Serial lighting interface with embedded feedback

ABSTRACT

A system for controlling multiple strings of LEDs includes a group of LED driver ICs, each of which includes a current sense feedback (CSFB) sample latch for storing a digital representation of the forward-voltage drop across a controlled LED string. Each CSFB latch is coupled to a register within a serial lighting interface (SLI) bus that both originates and terminates at an interface IC. As the data on the SLI bus is shifted into the interface IC, the interface IC selects the CSFB word that represents the highest forward-voltage drop of any of the controlled LED strings, which is then used by the interface IC to generate a CSFB signal for setting the appropriate supply voltage for the controlled LED strings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Provisional Application No.61/568,545, filed Dec. 8, 2011, which is incorporated herein byreference in its entirety.

This application is related to the following applications, each of whichis incorporated herein by reference in its entirety: application Ser.No. 13/346,625, filed Jan. 9, 2012, entitled Low Cost LED Driver withIntegral Dimming Capability; application Ser. No. 13/346,647, filed Jan.9, 2012, entitled Low Cost LED Driver with Improved Serial Bus.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor devices and circuits and methodsfor driving LEDs in lighting and display applications.

LEDs are increasingly being used to replace lamps and bulbs in lightingapplications including providing white light as a backlight in colorliquid crystal displays (LCD) and high definition televisions (HDTV). Tobacklight a color LCD panel, the LED strings may comprise white LEDs orcombinations of red, green, and blue LEDs adjusted to produce whitelight with controllable color temperature. While these LEDs may be usedto uniformly light the entire display, the performance, contrast,reliability, and power efficiency of the display are improved byemploying multiple strings of LEDs, each driven to different current andbrightness levels corresponding to that portion of the display theparticular LED string illuminates. The term “local dimming” refers tobacklighting systems capable of such non-uniform backlight brightness.Power saving can be as high as 50% over that of LCDs employing uniformbacklighting. Using local dimming, LCD contrast ratios can approach thatof plasma TVs.

To control the brightness and uniformity of the light emitted from eachstring of LEDs, special electronic driver circuitry must be employed toprecisely control the LED current and voltage. For example, a string of“m” white LEDs connected in series requires a voltage equal toapproximately 3.1 to 3.5 (typically 3.3) times “m” to operateconsistently. Supplying this requisite voltage to a LED string generallyrequires a step-up or step-down voltage converter and regulator called aDC-to-DC converter or switch-mode power supply (SMPS). When a number ofLED strings are powered from a single SMPS, the output voltage of thepower supply must exceed the highest voltage required by any of thestrings of LEDs. Since the highest forward voltage required cannot beknown a priori, the LED driver IC must be intelligent enough todynamically adjust the power supply voltage using feedback. If two ormore power supply voltages are required, more than one feedback signalis required.

In the case of RGB backlighting, the voltage feedback requirement iseven more complex because red, green and blue LEDs have significantlydifferent forward voltages and cannot share a common power supply rail.Instead, RGB LED strings require three different supply voltages,+V_(RLED), +V_(GLED), and +V_(BLED), respectively, each with separatefeedback signals to dynamically adjust their respective power supplyvoltages to the proper level. For example, a string of 30 red LEDs inseries requires a supply over 66V to operate properly, while 30 blueLEDs may require a supply over 96V, and 30 green LEDs requires a supplyof more than 108V.

In addition to providing the proper voltage to the LED strings, thebacklight driver must precisely control the current I_(LED) conducted ineach string to a tolerance of ±2%. Accurate current control is necessarybecause the brightness of an LED is proportional to the current flowingthrough it, and any substantial string-to-string current mismatch willbe evident as a variation in the brightness of the LCDs. Aside fromcontrolling the current, local dimming requires precise pulse control ofLED illumination, both in timing and duration, in order to synchronizethe brightness of each backlight region, zone, or tile to thecorresponding image in the LCD screen.

Another complication is that the color temperature of white LEDs varieswith current. As an example, a string of white LEDs conducting 30 mA for100% of the time is ideally equivalent in brightness to the same LEDstring carrying 60 mA pulsed on and off at a 50% duty factor. Even atthe same brightness, however, the color temperature will not be thesame. So accurately setting and maintaining current in each string iscritical to achieving a uniform white backlight for a color LCD panel.

In the case of RGB backlighting, balancing current is even more complex,since the luminosity, i.e. the light output or brightness, of red, blueand green LEDs differs substantially. Red LEDs, for example, produceless light for the same LED current than blue LEDs. The difference isunderstandable since the semiconductor materials and the manufacturingprocess used to make LEDs of different colors differ significantly.

As will be shown in this background section, the known solutions tolocal dimming limit display brightness and suffer from high solutioncosts. For example, early attempts to integrate LED driver controlcircuitry with multiple channels of high-voltage current sinktransistors were problematic because a mismatch in the forward-voltageof the LED strings resulted in excessive power dissipation andoverheating. Attempts to minimize power dissipation by lowering LEDcurrents and limiting the number of LEDs in a string (for betterchannel-to-channel voltage matching) proved uneconomical, requiring moreLEDs and a greater number of channels of LED drive. As a result, thefully integrated approach to LED backlight drive has been limited tosmall display panels or very expensive “high-end” HDTVs.

Subsequent attempts to reduce overall display backlight costs usingmultichip approaches have sacrificed necessary features, functionality,and even safety.

For example, the multichip solution to driving LEDs shown in FIG. 1comprises an interface IC 6 driving multiple discrete current sinkDMOSFETs 4 and high-voltage protection devices 3. The backlight systemcomprises sixteen LED strings 2A-2Q (referred to collectively as LEDstrings 2) with each of LED strings 2A-2Q containing “m”series-connected LEDs, ranging in length from 2 to sixty LEDs. (Notethat the letter “O” has been omitted in the series 2A-2Q to avoidconfusion with the number zero.) Each LED string has its currentcontrolled by one of discrete current sink DMOSFETs 4A-4Q, respectively.Interface IC 6 sets the current in each LED string in response toinstructions from a backlight microcontroller (μC) 7 communicatedthrough a high speed, expensive, SPI bus interface 11. MicrocontrollerμC 7 receives video and image information from a scalar IC 8 in order todetermine the proper lighting levels needed for each LED string.

As shown, every LED string 2A-2Q is powered by a common LED power supplyrail 12, generated by a switch-mode power supply (SMPS) 9, having avoltage +V_(LED) generated in response to a current-sense feedback(CSFB) signal 10 through feedback from interface IC 6. Supply voltagesvary with the number of LEDs “m” connected in series and may range from35 volts for ten LEDs up to 150 volts for strings of 40 LEDs. SMPS 9 maybe powered from the AC mains or alternatively from another input such asa +24V input.

SMPS 9 typically comprises a flyback converter operating in hardswitching or in quasi-resonant mode. Forward converters and Cukconverters, while applicable, are generally too expensive andunnecessarily complex to serve the cost-sensitive display and TVmarkets. In the event that SMPS 9 is powered from a +24V input, itsoperation depends on the number of LEDs connected in series. If theforward voltage of the LED string is less than 24V, e.g. a seriesconnection of less than 7 LEDs, SMPS 9 can be realized using a Buck-typeswitching regulator. Conversely, if the forward voltage of the LEDstring is greater than 24V, e.g. a series connection of more than 8LEDs, then SMPS 9 can be realized using a boost-type switchingregulator.

Regardless of its input voltage, the proper generation of the CSFBsignal 10 is critical to achieving reliable operation for a display'sLED backlight. If the feedback signal is incorrect, the LED supplyvoltage +V_(LED) may be too high or too low. If the LED supply voltageis too high, excess power dissipation will occur in the current sinkDMOSFETs 4A-4Q. If the LED supply voltage is too low, the LED stringsrequiring the highest current will not illuminate at the prescribedlevel, if at all.

To implement the CSFB function, accurate monitoring of a LED string'sforward-voltage requires electrical access to the drain of the currentsink DMOSFETs 4A-4Q, which for multichip implementations can beparticularly problematic, resulting in additional package pins and addedcomponent cost.

Current sink DMOSFETs 4A-4Q are realized using discrete DMOSFETs toavoid overheating. Additional discrete MOSFETs 3A-3Q, typically highvoltage discrete MOSFETs, are optionally employed to clamp the maximumvoltage present across the current sink DMOSFET 4, especially foroperation at higher voltages, e.g. over 100V.

Each of components 3A-3Q is a discrete device in a separate package,requiring its only pick-place operation to position and mount it on itsprinted circuit board. A current sink DMOSFET, a clamping MOSFET (ifany), and its associated LED string are commonly referred to as a“channel.”

Each set of discrete MOSFETs 3A-3Q and DMOSFETs 4A-4Q, along with itscorresponding string of white LEDs, is repeated “n” times for ann-channel LED driver system. For example, in addition to SMPS module 9,a 16-channel backlight system requires 34 components, namely amicrocontroller, a high-pin-count LED interface IC, and 32 discreteMOSFETs, to facilitate local dimming in response to video informationgenerated from scalar IC 8. The solution is complex and expensive.

In some cases, it is desirable to split LED power into more than onepower supply, e.g. to reduce the power dissipation in any one supply andits components, but prior art LED interface ICs cannot support multipleindependent feedback signals. In the case of RGB backlit displays, thesolution is even more complex and expensive. Since existing and priorart LED drivers and controllers include only a single CSFB signal perintegrated circuit, independently regulating three different powersupplies requires three separate LED interface ICs along with threeseparate power supplies, making today's RGB backlighting solutionsprohibitively expensive.

In either case, the assembly of a large number of discrete components,i.e. a high build of materials (BOM) count, results in expensive PCBassembly, further exacerbated by the high package cost of high pin countpackage 6. The need for such a large number of pins is illustrated inFIG. 2A, illustrating greater circuit detail for an individual channelof an LED drive system. As shown, each channel includes a string of “m”series connected LEDs 21, a cascode-clamp MOSFET 22 with an integralhigh-voltage diode 23, a current sink MOSFET 24, and a current-sensingI-Precise gate driver circuit 25.

The active current sink MOSFET 24 is a discrete power MOSFET, preferablya vertical DMOSFET, having a gate, source and drain connection.I-Precise gate driver circuit 25 senses the current in current sinkMOSFET 24 and provides it with the requisite gate drive voltage toconduct a precise amount of current. In normal operation, current sinkMOSFET 24 operates in its saturated mode of operation, controlling aconstant level of current independent of its drain-to-source voltage. Asa result of the simultaneous presence of drain voltage and current,power is dissipated in MOSFET 24.

Continuous measurement of the drain voltage of current sink MOSFET 24 isrequired for two purposes—to detect the occurrence of shorted LEDs in anLED fault circuit 27 and to facilitate feedback to the system's SMPSthrough CSFB circuit 26. The signal generated by CSFB circuit 26 iscritical to dynamically adjust +V_(LED) to the proper voltage, highenough to guarantee every LED string is illuminated but low enough toavoid excess voltage impressed upon the current sink DMOSFET 24resulting in unwanted power dissipation. With only one CSFB signal, itis not possible to power the LEDs from more than one power supply, i.e.to split the power requirements in two to reduce the size, cost andheating in the SMPS.

Current sink MOSFET 24 requires three connections to the control IC,specifically the source for current measurement, the gate for biasingthe device, and the drain for fault and feedback sensing. These threeconnections per channel are depicted crossing the discrete-to-ICinterface 28. Even in FIG. 2B where a cascode clamp MOSFET 22 iseliminated and current sink MOSFET 24 must sustain high voltages,illustrated by HIV integral diode 23, each channel still requires threepins per channel crossing interface 28. This three-pin per channelrequirement explains the need for high-pin count interface IC 6 shown inFIG. 1. For a sixteen-channel driver, the need for three pins perchannel uses 48 pins for the outputs. Including the SPI bus interface,analog functions, power supplies and more, a costly 64 or 72-pin packageis necessary. Worse yet, many TV printed circuit board assembly housesare incapable of soldering packages with a pin pitch any smaller than0.8 or 1.27 mm. A 72-pin package with a 0.8 mm pin pitch requires a14×14 mm plastic body to accommodate the peripheral linear edge neededto fit all the pins.

One significant issue with the multichip structure shown in FIG. 1 isthat temperature sensing circuitry in interface IC 6 can only detect thetemperature of the interface IC itself, where no significant powerdissipation is occurring. Unfortunately, the significant heat is beinggenerated in discrete current sink DMOSFETs 4, where no temperaturesensing is possible. Without temperature sensing, any one of the currentsink MOSFETs 4A-4Q could overheat without the system being able todetect or remedy the condition.

In summary, today's implementations for LED backlighting of LCD panelswith local dimming capability suffer from numerous fundamentallimitations in cost, performance, features, and safety.

Highly integrated LED driver solutions require expensive large area dicepackaged in expensive high pin count packages, and concentrate heat intoa single package, limiting the driver to lower currents, due to powerdissipation resulting from the linear operation of the current sinkMOSFETs, and lower voltages, due to power dissipation resulting from LEDforward-voltage mismatch, exacerbated for greater numbers of seriesconnected LEDs.

Multi-chip solutions combining an LED controller with discrete powerMOSFETs require BOM counts and even higher-pin-count packaging. Havingnearly triple the pin count of fully integrated LED drivers, a sixteenchannel solution can require 33 to 49 components and a 72 pin package aslarge as 14 mm×14 mm. Moreover, discrete MOSFETs offer no thermalsensing or protection against overheating. With only one feedbacksignal, these LED drivers cannot power two or more LED power supplieswithout including additional interface ICs, adding cost and complexity.

Similarly, expanding the use of these existing LED driver and interfaceICs to RGB backlighting requires even a higher BOM count, includingthree large high-pin count packages and all the associated discreteMOSFETs.

What is needed for a cost effective and reliable backlight system forTV's with local dimming is a new semiconductor chip set that eliminatesdiscrete MOSFETs, provides a low overall package cost, minimizes theconcentration of heat within any component, facilitates over-temperaturedetection and thermal protection, protects low voltage components fromhigh voltages and against shorted LEDs, flexibly scales to accommodatedifferent size displays, and maintains precise control of LED currentand brightness.

Ideally, a flexible solution would be scalable to accommodate a varyingnumber of channels, feedback signals, power supplies, and display panelsof different sizes without requiring custom integrated circuits.

BRIEF SUMMARY OF THE INVENTION

A system according to this invention comprises an interface integratedcircuit (IC) that transmits a current sense feedback (CSFB) signal to aswitch-mode power supply (SMPS) module for setting a single supplyvoltage for a plurality of light-emitting diode (LED) strings. Aplurality of LED driver ICs control the currents in and provide otherfunctions for the LED strings.

Each LED driver IC controls at least two LED strings (channels) andcomprises a serial lighting interface (SLI) bus shift register which isfunctionally linked to latches in the LED driver IC. The latches areused to store digital data which controls the currents in the LEDstrings and which can be used to control or monitor other functions withrespect to the LED strings, such as detecting short- and open-circuitsin the LED string and excessive temperatures in the LED driver IC.

In accordance with the invention, each LED driver IC comprises circuitrywhich periodically samples the forward-voltage drop across eachcontrolled LED string and a CSFB sample latch for storing a digitalrepresentation of such samples. The CSFB latch may store a digitalrepresentation of the highest forward-voltage drop in any of the LEDstrings that are controlled by the LED driver IC. Each CSFB latch iscoupled to a register within the SLI bus shift register of the LEDdriver IC. In some embodiments, the forward-voltage drop across an LEDstring is detected by sensing the voltage at a current sink MOSFET whichcontrols the current through an LED string.

The respective SLI bus shift registers in the LED driver ICs areconnected together serially by SLI bus lines in a “daisy chain”arrangement, thereby forming an SLI bus which both originates andterminates at the interface IC. Thus data bits shifted out of theinterface IC move serially through the SLI bus shift registers and theSLI bus lines connecting them and return to the interface IC, in theprocess pushing data bits that were previously stored in SLI busregisters back to the interface IC in a serial manner.

In some embodiments, the SLI bus shift register contains a dedicatedCSFB register that stores the CSFB data received from the CSFB latchwithin the LED driver IC. Similarly, the SLI bus shift register maycontain other dedicated registers that are linked in a one-to-onerelationship with other functional and sampling latches within the LEDdriver IC.

Alternatively, in a preferred embodiment, the SLI bus shift register ineach LED driver IC comprises a prefix register and a data register, i.e,the one-to-one relationship between the registers in the SLI busregister and the latches in the LED driver IC. The CSFB latch and otherlatches in the LED driver IC are identified by a digital word (address)in the prefix register, allowing the data stored in the CSFB latch to becopied into the data register. This structure saves valuablesemiconductor “real estate” on the LED driver IC and thereby reducescosts significantly.

In either embodiment, a digital word representing the highestforward-voltage of any LED string controlled by the LED driver IC can beread into and stored in a register within the driver IC's SLI bus shiftregister.

The interface IC contains circuitry that is capable of receiving theCSFB data stored in the SLI bus shift registers and selecting there fromthe CSFB word that represents the highest forward-voltage drop of any ofthe LED strings controlled by the LED. This word is then used by theinterface IC to generate a CSFB signal which the SMPS module uses to setthe appropriate supply voltage for all of the controlled LED strings.

The sampling of CSFB data in the LED driver ICs is carried out atpredetermined intervals, after each of which the data is again shiftedto the interface IC, which then sends a new SCFB signal to the SMPSmodule, allowing the supply voltage to be adjusted appropriately basedon the new highest forward-voltage drop among the controlled LEDstrings.

This arrangement allows flexibility and scalability for systems thatcontain different numbers of LED strings and other parameters.

The arrangement also allows an LED drive system to easily be partitionedinto different groups of LED strings that are supplied by different SMPSmodules at different supply voltages. This is useful, for example, whena display contains separate strings of red, green and blue LEDs, whichtypically require different supply voltages. In this situation theinterface IC contains circuitry that is capable of separating the CSFBwords that relate to the red, green and blue LED strings, respectively,and sending appropriate CSFB signals to separate SMPS modules thatprovide supply voltages for the different LED strings. In embodimentswherein the SLI bus shift registers contain dedicated SCFB registers theseparation process can be performed by counting the number of bitsreceived to identify which of the LED strings the CSFB data relates to.In embodiments wherein the SLI shift registers contain prefix and dataregisters, the prefix can be used to identify which of the LED stringsthe CSFB data relates to.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagram of a prior-art multi-channel LED drive system forLCD backlighting using discrete DMOSFETs as integrated current sinks andprotective voltage clamps.

FIG. 2A is a schematic diagram of an individual LED drive channel usingdiscrete current sink DMOSFETs and protective high-voltage cascode clampDMOSFETs.

FIG. 2B is a schematic diagram of an individual LED drive channel usingdiscrete high-voltage current sink DMOSFETs without cascode clampMOSFETs.

FIG. 3A is a schematic diagram of a dual-channel high-voltageintelligent LED driver with serial bus control with protectivehigh-voltage cascode clamp DMOSFETs and a fat SLI bus interface.

FIG. 3B is a schematic diagram of a dual-channel high-voltageintelligent LED driver with serial bus control using high-voltagecurrent sink MOSFETs without cascode clamp MOSFETs and fat SLI businterface.

FIG. 3C is a schematic diagram of a dual-channel high-voltageintelligent LED driver with serial bus control using high-voltagecurrent sink MOSFETs without cascode clamp MOSFETs and comprising aprefix multiplexed SLI bus interface.

FIGS. 4A and 4B illustrate a diagram of a multi-channel LED backlightsystem using intelligent LED drivers without cascode-clamp MOSFETs andcomprising an SLI serial bus control.

FIG. 5 is a simplified schematic diagram of the system shown in FIG. 4,illustrating the significantly reduced build-of-materials.

FIG. 6 is a schematic diagram illustrating SLI bus-based control of anintelligent backlighting system with embedded SLI bus control.

FIG. 7A is a block diagram of a dual-channel LED driver withcorresponding digital control and timing (DC&T) and analog control andsensing (AC&S) circuitry with embedded CSFB feedback and using a fat SLIbus interface and protocol.

FIG. 7B is a block diagram of a dual-channel LED driver withcorresponding digital control and timing (DC&T) and analog control andsensing (AC&S) circuitry with embedded CSFB feedback and using a prefixmultiplexed SLI bus.

FIG. 8 is a block diagram illustrating a multi-function prefixmultiplexed SLI bus register with a corresponding three-tieredregister-latch architecture including preload and active latches andembedded CSFB capability.

FIG. 9 is a simplified diagram illustrating the communication pathway ofan SLI bus-embedded CSFB signal from an LED driver IC to an interfaceIC.

FIG. 10A is a schematic diagram of a dual-channel high-voltageintelligent LED driver with SLI bus embedded CSFB and serial bus controlwith a protective high-voltage cascode clamp DMOSFET and fat SLI bus.

FIG. 10B is a schematic diagram of a dual-channel high-voltageintelligent LED driver with SLI bus embedded CSFB and serial bus controlusing high-voltage current sink MOSFETs without cascode clamp MOSFETsand comprising a fat SLI bus.

FIG. 10C is a schematic diagram of a dual-channel high-voltageintelligent LED driver with SLI bus embedded CSFB and serial bus controlusing high-voltage current sink MOSFETs without cascode clamp MOSFETsand comprising a prefix multiplexed SLI bus.

FIGS. 11A and 11B illustrate a multi-channel LED backlight system usingintelligent LED drivers with cascode-clamp MOSFETs and a SLIbus-embedded CSFB signal.

FIG. 12 illustrates an embodiment of an analog CSFB circuit.

FIG. 13 illustrates an embodiment of an analog-to-digital CSFB converter

FIG. 14 illustrates an embodiment of a digital-to-analog CSFB converter.

FIG. 15 is a schematic diagram illustrating SLI bus-based control of anintelligent backlighting system with an embedded SLI bus control fordual power supplies.

FIG. 16 is a block diagram of an interface IC for controlling dual powersupplies using fat SLI bus protocol.

FIG. 17 is a block diagram of an interface IC for controlling dual powersupplies using a prefix multiplexed SLI bus protocol and prefix-specificCSFB signals.

FIG. 18 is a block diagram of a quad-CSFB SLI-bus protocol and decodingsystem using a single SLI bus prefix.

FIG. 19 is a block diagram of an interface IC for controlling multiplepower supplies using an alternative quad-CSFB encoded prefix-multiplexedSLI bus protocol.

FIG. 20 is a schematic diagram illustrating SLI bus-based control of anintelligent RGB backlighting system with embedded SLI bus control fortriple power supplies with one-color per driver IC partitioning.

FIG. 21 is a schematic diagram illustrating SLI bus-based control of anintelligent RGB backlighting system with embedded SLI bus control fortriple power supplies with three-color per driver IC partitioning.

FIG. 22 illustrates an eight-channel LED driver integrating eightcurrent sink DMOSFETs and four independent CSFB detect circuits forproviding independent feedback control to generate four distinct supplyvoltages.

DETAILED DESCRIPTION OF THE INVENTION

As described in the background section, existing backlight solutions forTVs and large screen LCDs are complex, expensive and inflexible. Toreduce the cost of backlight systems for LCD's with local dimmingwithout sacrificing safe and reliable operation clearly requires acompletely new architecture that in the very least eliminates discreteMOSFETs, minimizes the concentration of heat within any component,facilitates over-temperature detection and thermal protection, andprotects low voltage components from high voltages. While meeting theseobjectives may alone be insufficient to achieve a truly cost-effectivesolution able to meet the demanding cost targets of the home consumerelectronics market, such an improvement is a necessary first step towardsuch a goal toward realizing low-cost local dimming.

This invention described herein enables a new cost-efficient andscalable architecture for realizing safe and economically viable LEDbacklighting systems for large-screen LCDs and TVs with energy efficientlocal dimming capability. The new LED drive system, functionalpartitioning, and architecture, disclosed herein, completely eliminatethe aforementioned problems in cost, functionality and the need for highpin count packages. The new architecture is based on certain fundamentalpremises including

-   -   1. The analog control, sensing, and protection of the current        sink MOSFETs should be functionally integrated together with        their associated current sink MOSFETs, not separated into        another IC    -   2. Basic dimming, phase delay functions, LED current control and        channel specific functions should be functionally integrated        together with the current sink MOSFETs they control, not        separated into another IC    -   3. System timing, system μC host negotiations, and other global        parameters and functions not unique to a specific channel should        not be functionally integrated together with the current sink        MOSFETs    -   4. The number of integrated channels, i.e. current sink MOSFETs,        per packaged device should be optimized for thermal management        to avoid overheating while meeting specified LED current, supply        voltage and LED forward-voltage mismatch requirements    -   5. Communication with and control of multi-channel LED drivers        should employ a low-pin count method, ideally requiring no more        than three package pins in total on the central interface IC as        well as on each LED driver IC. The communication method should        constitute only a small fraction of the driver IC's die area and        cost.    -   6. The level of functional integration in the interface and        driver ICs should be balanced to facilitate the use of low-cost        and low-pin count packages compatible with single layer PCB        assembly    -   7. Ideally, the system should flexibly scale to any number of        channels without requiring significant redesign of the ICs

The conventional architecture of FIG. 1, i.e. a centralized controllerdriving a number of discrete power MOSFETs, fails to meet even one ofthe above goals, primarily because it requires a central point ofcontrol, or “command center”, for all digital and analog informationprocessing. Necessarily, the command center IC must communicate with itsμC host as well as directly sensing and driving every current sinkMOSFET. This high degree of component connectivity demands a largenumber of input and output lines, necessitating high pin countpackaging.

Distributed LED Drive Architecture Overview

In sharp contrast to the prior art, the above design criteria describe,if not mandate, a “distributed” system, one lacking the need for centralcontrol. In the disclosed distributed system, an interface IC translatesinformation obtained from the host μC into a simple serialcommunications protocol, sending instructions digitally to any number ofintelligent LED driver “satellite” ICs connected to the serial bus.

The implementation of an LED driver facilitating the above criteria isdescribed in application Ser. No. 13/346,625, to Williams et al.,entitled “Low Cost LED Driver with Integral Dimming Capability” byWilliams et al., which is incorporated herein by reference in itsentirety. An alternative version of an LED driver is described inapplication Ser. No. 13/346,647, to Williams et al., entitled “Low CostLED Driver with Improved Serial Bus” by Williams et al., which is alsoincorporated herein by reference in its entirety.

The main concepts of these applications are reiterated here, including ahardware description of the interface and LED driver ICs, and theoperation of several versions of an inventive “serial lightinginterface” or SLI bus—serial communication protocols containingparameters specifically relevant to controlling LED lighting. Eachdriver IC, in response to its SLI bus digital instructions, performs allthe necessary LED driver functions such as dynamic precision LED currentcontrol. PWM brightness control, phase delay, and fault detection,locally, without the assistance of the interface IC. When connected in a“daisy-chain” back to the interface IC, fault conditions such as an openLED, a shorted LED, or an over-temperature fault occurring in any of thedriver ICs can also communicated back to the interface IC and ultimatelyto the host μC.

While the basic architecture disclosed in the two above-referencedapplications is similar, their realization of the SLI bus protocol andphysical interface differs. In the “fat” SLI bus protocol described inthe first application, long digital words are employed to load all thecontrol parameters into every LED driver IC in a single SLI busbroadcast, i.e. all the data for every register of every driver IC isshifted out of the interface IC onto the SLI bus at once. In thealternative version described in the second application, the inventive“prefix multiplexed” SLI bus protocol, smaller digital words aredirected toward specific functional latches using multiple SLI busbroadcasts.

Regardless of the SLI bus protocol employed, each LED driver IC includesan analog current sense feedback (CSFB) input and output pins (CSFB1 andCSFBO), connected in a daisy chain with CSFB1 and CSFBO pins of theother driver ICs and with the interface IC to provide feedback to thehigh-voltage switch-mode power supply (SMPS), dynamically regulating thevoltage powering the LED strings. The analog CSFB signals require twopackage pins on every LED driver IC. Each LED driver IC outputs only asingle CSFB signal regardless the number of integrated channels.

Through the SLI bus, every satellite LED driver-IC communicates with acentral companion interface IC, interpreting SPI bus commands from avideo/graphics processor or scalar IC and translating the SPI businformation it receives into SLI bus commands. Along with itstranslation responsibilities, this interface IC supplies a referencevoltage to all the LED-driver ICs needed to insure good currentmatching, generates Vsync and grey scale clock (GSC) pulses tosynchronize their operation, and monitors every LED driver IC forpotential faults. It also facilitates voltage-to-current translation ofthe analog voltage CSFB signal into an analog current CSFB signal, usingan on-chip operational transconductance amplifier (OTA). The analog CSFBsignals require two package pins (CSFBI and ICSFB) on the interface IC,the CSFBI pin to receive the voltage CSFB signal from the LED driver ICsand the ICSFB pin to transmit the current CSFB signal to the SMPS.

So by repartitioning the functions of a LED backlight system such thatprecision current control and dimming, fault detection and CSFB sensingand feedback are integrated with the high voltage current sink DMOSFETsand not in the system interface IC, high pin count packages can beeliminated and a scalable distributed system is realized.

LED Drivers with Integral Dimming, Fault Detection and CSFB Feedback

The implementation of an inventive LED driver IC 51 with SLI buscommunication is shown in FIG. 3A comprising a dual-channel driver withintegrated current sink DMOSFETs 55A and 55B, cascode clamp DMOSFETs 57Aand 57B with integral high-voltage diodes 58A and 58B, I-precise gatedriver circuits 56A and 56B for accurate current control, an analogcontrol and sensing circuit 60, and a digital control and timing circuit59. An on-chip bias supply and regulator 62 powers the IC.

LED driver IC 51 provides complete control of two channels of 250 mA LEDdrive with 150V blocking capability and ±2% absolute current accuracy,12 bits of PWM brightness control, 12 bits of PWM phase control, 8 bitsof current control, fault detection for LED open and LED shortconditions and over-temperature detection, all controlled through ahigh-speed SLI bus, and synchronized to other drivers by a common Vsyncand grey-scale clock (GSC) signal. While the specific example shownillustrates cascode clamp DMOSFETs 57A and 57B rated at 150V blockingcapability, the devices can be sized for operation from 100V to 300V asneeded. The driver IC's current rating of 250 mA is set by the powerdissipation of the package and the mismatch in forward voltage in thetwo LED strings 52A and 52B.

In operation, LED driver IC 51 receives a stream of data on its serialinput SI pin which is fed into the input of an SLI bus shift register61. The data is clocked at a rate set by a serial clock SCK signalsupplied by the interface IC (not shown). The maximum clock rate for thedata depends on the CMOS technology used to implement shift register 61,but operation at 10 MHz is achievable even using 0.5 μm line-widthprocesses and water fabs. As long as the SCK signal continues to run,data will shift into shift register 61 and ultimately exit the serialout pin SO on its way to the next LED driver IC in the serial daisychain.

After the data intended for the driver IC 51 arrives in shift register61, the SCK signal is momentarily stopped by the interface IC sendingthe data. Using the “fat” SLI bus protocol, all the serial data used tocontrol the LED driver IC parameters are shifted into SLI bus shiftregister 61 at one time, i.e., the data is shifted into the shiftregister 61 in every driver IC in the daisy chain. Even if only oneparameter must be changed, all the data is rewritten into the shiftregisters 61. Thereafter, a Vsync pulse latches the data from the shiftregister 61 into latches contained within the digital timing and control(DC&T) circuit 59 and into latches contained within analog control andsensing AC&S circuit 60, these latches comprising flip flops or staticRAMs. Also at the time of the Vsync pulse, any data previously writteninto the fault latches contained within AC&S circuit 60 will be copiedinto the appropriate bits of SLI bus shift register 61.

A resumption of the serial clock signal SCK will move the read and thewrite bits within shift register 61 on through the daisy chain into thenext IC. In a preferred embodiment, the daisy chain forms a loopconnecting back to the interface IC. Sending new data into the daisychain ultimately pushes the existing data residing in the shiftregisters on through the loop and ultimately back to the interface IC.In this manner the interface IC can communicate with the individual LEDdriver ICs setting LED string brightness and timing, and the individualdriver ICs can communicate individual fault conditions back to theinterface IC.

Using this clocking scheme, data can be shifted through a large numberof driver ICs at a high speed without affecting the LED current orcausing flicker, because the current and timing controlling the currentsink DMOSFETs 55A and 55B only change upon each new Vsync pulse. Vsyncmay vary from 60 Hz to 960 Hz with the grey scale clock frequencyscaling proportionately, typically 4096 times the Vsync frequency. SinceVsync is slow, under 1 kHz, when compared to the SLI bus clock SCKfrequency, the interface IC has flexibility to modify and resend thedata, or query the fault latch multiple times within a givenvertical-sync pulse duration.

Commencing on the Vsync pulse, the DC&T circuit 59 generates two PWMpulses to toggle the output of I-Precise gate driver circuits 56A and56B on and off after the proper phase delay and for the properpulse-width duration, or duty factor D. I-Precise gate driver circuits56A and 56B sense the current in current sink MOSFETs 55A and 55Brespectively and provide the proper gate drive voltage to maintain atarget current during the time each current sink MOSFET is enabled bythe PWM pulse provided by each I-Precise gate driver circuit. Operationof the I-Precise gate driver circuit is therefore that of a “strobed”amplifier, being pulsed on and off digitally but controlling the currentin the LEDs as an analog parameter.

The peak current is set globally for all the LED driver ICs by the Vrefsignal and by the value of an Iset resistor 54. The Vref signal is, in apreferred embodiment, generated by the interface IC, or it may besupplied as an auxiliary output from the SMPS.

The specific current in any one LED string can be further controlledthrough the SLI bus by the Dot latch, comprising an 8 to 12 bit word,that adjusts the current sink MOSFET's current to a percentage from 0%to 100% of the peak current value in either 256 to 4096 different steps,respectively. In this manner, precise digital control of the LEDcurrent, emulating the function of a current mode digital-to-analogconverter or “current DAC”, is possible using the newly disclosedarchitecture. In LCD backlighting applications this feature can be usedfor calibrating the backlight brightness, for improving backlightuniformity, or for operating in 3D mode.

As shown, the current flowing through LED string 52A is controlled bycurrent sink DMOSFET 55A and corresponding I-Precise gate driver circuit56A. Similarly, the current flowing through LED string 52B is controlledby current sink DMOSFET 55B and corresponding I-Precise gate drivercircuit 56B. The maximum voltage impressed upon current sink DMOSFETs55A and 55B is limited by cascode clamp DMOSFETs 57A and 57B. So long asthe number of LEDs “m” is not too large, the voltage +V_(LED) will notexceed the breakdown voltage of PN diodes 58A and 58B, and the maximumvoltage on the current sink MOSFETs will be limited to around 10V, onethreshold voltage below the gate bias impressed on cascode clampDMOSFETs 57A and 58B by bias circuit 62. Bias circuit 62 also generatesthe five-volt Vcc supply to operate its internal circuitry from the 24VVIN input using a linear voltage regulator and filter capacitor 53.

The drain voltages on current sink DMOSFETs 55A and 55B are alsomonitored by AC&S circuit 60 and compared to an over-voltage valuestored in its SLED register from SLI bus shift register 61. If the drainvoltage is below the programmed value, the LED string is operatingnormally. If, however, the voltage rises about the prescribed value, oneor more LED is shorted and a fault is detected and recorded for thatspecific channel. Likewise, if the I-Precise gate driver circuit cannotmaintain the required current, i.e. the LED string is operating“undercurrent”, it means an LED has failed open and the circuitcontinuity is lost. The channel is then turned off, its CSFB signal isignored, and the fault is reported. Sensing this “undercurrent”, can beperformed by monitoring the current sink DMOSFET for a saturationcondition, meaning that the I-Precise gate driver circuit is driving thegate of the current sink DMOSFET as “full on” as it can, oralternatively by monitoring the voltage drop across the input terminalsof I-Precise gate driver circuit. When the voltage at the inputterminals of the I-Precise gate driver circuit drops too low, theundercurrent condition has occurred, thereby indicating an open-LEDfault.

If an over-temperature condition is detected, a fault is reported andthe channel is left on and conducting unless the interface IC sends acommand to shut down that channel. If, however, the temperaturecontinues to rise to dangerous levels, AC&S circuit 60 will disable thechannel independently and report the fault. Regardless of the nature ofa fault, whether shorted LED, open LED, or over-temperature, whenever afault occurs an open drain MOSFET within AC&S circuit 60 will activateand pull the FLT pin low, signaling to the host μC that a faultcondition has occurred.

AC&S circuit 60 also includes an analog current sense feedback (CSFB)signal which reflects the voltages at the drains of the two current sinkDMOSFETs 55A and 55B and at the CSFB1 input pin to determine which ofthe three voltages is lowest and passing that voltage to the CSFBOoutput pin. In this way the lowest current sink MOSFET source voltage,and hence the LED string with the highest forward-voltage drop is passedto the input of the next driver IC and ultimately back to the interfaceIC, which responsively generates an CSFB signal which the SMPS uses tosupply the correct +V_(LED) to the supply rail for the LED strings. Theintegrated current sense feedback function uses two pins (CSFB1 andCSFBO) on each LED driver IC and outputs only one analog signal on theCSFBO pin, regardless of the number of channels integrated in LED driverIC 51.

In the manner described, an LED driver IC 51 with integral diming andfault detection capability can be realized without the need of a centralinterface IC.

An alternate implementation of an LED driver 65 meeting the abovecriteria is shown in FIG. 3B. LED driver 65, which is integrated in anLED driver IC 66, is a dual-channel driver with integrated current sinkDMOSFETs but without cascode clamp MOSFETs. Instead current sinkDMOSFETs 72A and 72B contain integral high-voltage diodes 73A and 73Bthat are designed to sustain high-voltages in when DMOSFETs 72A and 72Bare in an off condition. Typically, such a design is most applicable tooperation below 100V, but it can be extended to 150V if required. As inthe LED driver IC 51 of FIG. 3A, I-precise gate driver circuits 71A and71B facilitate accurate current control, controlled by an analog controland sensing circuit 70, and a digital control and timing circuit 74. Anon-chip bias supply and regulator 69 powers the LED driver IC 66, inthis case from Vcc, not from the 24V input as in driver IC 51. Asidefrom lacking cascode clamp DMOSFETs, driver IC 66 operates similar todriver IC 50, controlled through its SLI bus 75.

LED driver IC 66 includes an integrated current sense feedback functionusing two pins and outputs only one analog signal CSFBO regardless ofthe number of channels integrated.

An implementation of an inventive LED driver 80 using aprefix-multiplexed SLI bus is shown in FIG. 3C. LED driver 80 is a dualchannel driver and is formed on an LED driver IC 81. LED driver includescurrent sink DMOSFET 87A and 87B with integral high-voltage diodes 88Aand 88B, I-precise gate driver circuits 86A and 86B, an analog controland sensing (AC&S) circuit 85, and a digital control and timing (DC&T)circuit 89. An on-chip bias supply and regulator 84 powers the LEDdriver IC 81 from a Vcc input.

LED driver 80 provides complete control of two channels of 250 mA LEDdrive with 150V blocking capability and ±2% absolute current accuracy,12 bits of PWM brightness control, 12 bits of PWM phase control, 8 bitsof current control, fault detection for LED open and LED shortconditions and over-temperature detection, all controlled through ahigh-speed SLI bus, and synchronized to other drivers by a common Vsyncand grey-scale clock (GSC) signal. While the specific example shownillustrates current sink DMOSFETs rated at 150V blocking capability, thedevices can be sized for operation from 100V to 300V as needed. Thedevice's current rating of 250 mA is set by the power dissipation of thepackage and the mismatch in forward voltage in the two LED strings beingdriven. Above a 100V rating, it is advantageous to integrate highvoltage cascode clamp DMOSFETs (not shown) in series with current sinkDMOSFETs 87A and 87B, whereby current sink MOSFETs 87A and 87B do notrequire operation above the clamp voltage, i.e. above 12V.

In operation, LED driver IC 81 receives a stream of data on its serialinput SI pin and fed into the input of prefix multiplexed SLI bus shiftregister 90. The data is clocked at a rate set by serial clock SCKsignal supplied by the interface IC, not shown. The maximum clock ratefor the data depends on the CMOS technology used to implement shiftregister 90, but operation at 10 MHz is achievable even using 0.5 μmline-width processes and wafer fabs. As long as SCK signal continues torun, data will shift into shift register 90 and ultimately exit theserial out pin SO on its way to the next device in the serial daisychain.

After the data corresponding to the specific driver IC arrives in shiftregister 90, the SCK signal is momentarily stopped by the interface ICsending the data. A decoder 91 interprets the functional latch andchannel to be controlled and directs multiplexer 92 to connect the dataregister within SLI bus interface 90 to the appropriate functional latchwithin digital control and timing (DC&T) circuit 89 or analog controland sensing (AC&S) circuit 85.

Thereafter, a Vsync pulse latches the data from the data shift registerin SLI bus 90 into latches contained within either DC&T circuit 89 orAC&S circuit 85, the latches comprising flip flops or static RAM. In theevent that the decoder instructs the SLI bus to interrogate the faultlatch within AC&S circuit 85, then at the time of the Vsync pulse, anydata previously written into the fault latches contained within AC&Scircuit 85 will be copied into the appropriate bits of SLI bus shiftregister 90.

A resumption of the serial clock SCK signal moves the read and the writebits within shift register 90 on through the daisy chain into the nextIC. In a preferred embodiment, the daisy chain forms a loop connectingback to the interface IC sending the data. Sending new data into thedaisy chain ultimately pushes the existing data residing in the shiftregisters on through the loop and ultimately back to the interface IC.In this manner the interface IC can communicate to the individual LEDdriver ICs setting LED string brightness and timing, and the individualdriver ICs can communicate individual fault conditions back to theinterface IC.

Using this clocking scheme, data can be shifted through a large numberof driver ICs at a high speed without affecting the LED current orcausing flicker, because the current and timing controlling the currentsink DMOSFETs 87A and 87B only changes upon each new Vsync pulse. Vsyncmay vary from 60 Hz to 960 Hz with the grey scale clock frequencyscaling proportionately, typically 4096 times the Vsync frequency. SinceVsync is slow, under 1 kHz, when compared to the frequency of the serialclock SCK signal, the interface IC has flexibility to modify and resendthe data, or query the fault latch multiple times in the intervalbetween successive Vsync pulses.

Because in the prefix multiplexed or “slim” SLI bus protocol the dataregister within SLI bus interface 90 is not large enough to write to allthe functional latches within DC&T circuit 89 and AC&S circuit 85 from asingle SLI bus word or data packet, then the interface IC must sendmultiple SLI bus packets to the driver ICs to load all the latches. Thiscondition arises at start-up when all the functional latches are firstinitiated, or when the data in more than one functional latch must bechanged contemporaneously. If the data controlling the I-Precise gatedriver circuits 86A and 86B is allowed to change gradually in multiplesteps over several Vsync periods, e.g. first changing the Φ latch, thenchanging the D latch, then changing the Dot latch, etc., a viewer may beable to discern the step changes as flicker or noise in the video image.Several inventive solutions to circumvent this potential problem aredisclosed in the section “Simultaneously Loading Multiple FunctionalLatches” in the above-mentioned application Ser. No. 13/346,647, byWilliams et al., entitled “Low Cost LED Driver with Improved SerialBus”.

After the functional latch data has been loaded, commencing on the nextVsync pulse, DC&T circuit 89 generates two PWM pulses to toggle theoutput of I-Precise gate driver circuits 86A and 86B on and off afterthe proper phase delay and for the proper pulse width duration, or dutyfactor D. I-Precise gate driver circuits 86A and 86B sense the currentin current sink MOSFETs 87A and 87B respectively and provide the propergate drive voltage to maintain a target current during the time currentsink MOSFETs 87A and 87B are is enabled by the PWM pulses from I-Precisegate driver circuits 86A and 86B. Operation of the I-Precise gate drivercircuits is therefore similar to that of a “strobed” amplifier, beingpulsed on and off digitally but controlling the current in the LEDs asan analog parameter.

The peak current in all the LED driver circuits is set globally by theVref signal and by the value of Iset resistor 82. The Vref signal is, ina preferred embodiment, generated by the interface IC, or may besupplied as an auxiliary output from the SMPS. In an alternativeembodiment, channel-specific Dot correction can be eliminated, and Vrefcan be modulated to facilitate global current control of the LEDcurrents.

In drivers capable of channel-specific Dot correction, the current inany one LED string can be controlled through the SLI bus by the Dotlatch, preferably comprising an 8 to 12 bit word, that adjusts thecurrent in the current sink MOSFET's to a percentage from 0% to 100% ofthe peak current value in either 256 to 4096 different stepsrespectively. In this manner, precise digital control of the LEDcurrents, emulating the function of a current mode digital-to-analogconverter or “current DAC”, is possible using the newly disclosedarchitecture. In LCD backlighting applications this feature can be usedfor calibrating the backlight brightness, for improving backlightuniformity, or for operating in 3D mode.

The structure and operation of I-Precise gate driver circuits 86A and86B as well as AC&S circuit 85 are discussed in detail in theabove-referenced application Ser. No. 13/346,625, to Williams et al.,entitled “Low Cost LED Driver with Integral Dimming Capability”.

As shown, the current flowing through LED string 83A is controlled bycurrent sink DMOSFET 87A and corresponding I-Precise gate drive circuit86A. Similarly, the current flowing through LED string 83B is controlledby current sink DMOSFET 87B and corresponding I-Precise gate drivecircuit 86B. Without cascode clamp MOSFETs, the maximum voltageimpressed upon current sink DMOSFETs 87A and 87B is limited to operationbelow the breakdown voltage of high-voltage diodes 88A and 88B. Biascircuit 84 generates internal chip bias voltage from a 5V Vcc input.

The drain voltages on current sink DMOSFETs 87A and 87B are alsomonitored by AC&S circuit 85 and compared to an over-voltage valuestored in its SLED register from SLI bus 90. If the drain voltage isbelow the programmed value, the LED string is operating normally. If,however, the voltage rises about the prescribed value, one or more LEDis shorted and a fault is detected and recorded for that specificchannel. Likewise if the I-Precise gate driver circuit cannot maintainthe required current, i.e. the LED string is operating “undercurrent”,it means an LED has failed open and the circuit continuity is lost. Thechannel is then turned off, its CSFB signal is ignored, and the fault isreported. Sensing this “undercurrent”, can be performed by monitoringthe current sink DMOSFET for a saturation condition, meaning that theI-Precise gate driver circuit is driving the gate of the current sinkDMOSFET as “full on” as it can, or alternatively by monitoring thevoltage drop across input terminals of the I-Precise gate drivercircuit. When the voltage at the input terminals of the I-Precise gatedriver circuit drops too low, the undercurrent condition has occurred,thereby detecting an open LED fault.

If an over-temperature condition is detected a fault is reported and thechannel is left on and conducting unless the interface IC sends acommand to shut down that channel. If, however, the temperaturecontinues to rise to dangerous levels, AC&S circuit 85 will disable thechannel independently and report the fault. Regardless of the nature ofa fault, whether shorted LED, open LED, or over-temperature, whenever afault occurs an open drain MOSFET within AC&S circuit 85 will activateand pull the FLT pin low, signaling to the host μC that a faultcondition has occurred.

AC&S circuit 85 also includes an analog current sense feedback (CSFB)signal which reflects the voltages at the drains of the two current sinkDMOSFETs 87A and 87B and at the CSFB1 input pin to determine which ofthe three voltages is lowest and passing that voltage to the CSFBOoutput pin. In this way, the lowest current source voltage, and hencethe LED string with the highest forward-voltage drop is passed to theinput of the next LED driver and ultimately back to the SMPS to powerthe +V_(LED) supply rail. The integrated current sense feedback functionintegrated in LED driver IC 81 uses two pins and outputs only one analogsignal CSFBO regardless of the number of channels integrated.

In the manner described, a two-channel LED driver with integral dimingand fault detection capability 81 can be realized without the need of acentral interface IC.

SLI Bus Interface IC and System Application

System 100 in FIG. 4 illustrates the application of a distributed systemfor LED backlighting with local dimming implemented in the mannerprescribed by this disclosure. The figure illustrates an interface IC101, driving a series of LED driver ICs 81A-81H with integral dimmingand fault detection powered by a common SMPS 108.

Each of LED driver ICs 81A-81H (sometimes referred to hereinindividually as LED driver IC 81) may comprise the device shown in FIG.3C employing the prefix-multiplexed SLI bus protocol or mayalternatively employ a fat bus protocol such as the devices shown inFIG. 3A and FIG. 3B. Each driver IC 81 may likewise incorporate ahigh-voltage current sink DMOSFET as shown in FIGS. 3B and 3C oralternatively may integrate a cascode clamp protected current sinkDMOSFET such as shown in FIG. 3A. In the system 100, LED driver IC 81 isillustrated without multiplexer 92 and decoder 91 (shown in FIG. 3C)with the understanding that such functions are embedded within SLI Businterface 90 as needed, i.e. whenever a prefix multiplexed SLI busprotocol is utilized.

Five common signal lines 107, comprising three digital clock signals,one digital fault signal, and one analog reference voltage connectinterface IC 101 to every driver IC 81. A timing and control circuit 124generates the Vsync and GSC signals in sync with data from A host μC(not shown) received through an SPI bus interface 122. Timing andcontrol circuit 124 also monitors the FLT interrupt line to immediatelydetect a potential problem. A reference voltage source 125 provides areference voltage to the system globally in order to insure goodchannel-to-channel current matching. A bias supply circuit 126 powersinterface IC 101 from a VIN voltage supplied by a fixed +24V supply rail110 generated by SMPS 108. Bias supply circuit 126 also generates aregulated supply voltage Vcc, preferably 5V, to power LED drivers81A-81H. The Vcc supply voltage is filtered by a capacitor 102.

In this example, each LED driver 81A-81H comprises two channels ofhigh-voltage current control including current sink DMOSFETs 87A-87Qwith integral HV diodes 88A-88Q, I-Precise gate driver circuits 86A-86Q,DC&T circuits 89A-89H, AC&S circuits 85A-85H and serial SLI businterface shift registers 90A-90H. While the LED driver ICs 81, like thedriver IC 66 shown in FIG. 3B, lack a cascode clamp, the systemconfiguration works equally well with LED driver IC 51 shown in FIG. 3A,except that the 24V VIN supply, rather than Vcc, is used to power theLED driver ICs and bias the gates of the cascode clamp DMOSFETs.

An SLI bus 113, comprising lines 113A-113I connecting the LED drivers81, comprises a daisy chain, in the example shown where the SO serialoutput of SLI circuit 123 within interface IC 101 connects via line 113Ato the SI input of LED driver 81A, the SO output of LED driver 81Aconnects via line 113B to the SI input of LED driver 81B (not shown) andso on. SLI bus line 113H connects to the SI input of last LED driver 81Hshown in system 100. The SO output of LED driver 81H, in turn, connectsvia line 113I to the SI input of SLI circuit 123 within interface IC101. In this manner SLI bus lines 113A-113I (collectively referred to asSLI bus 113) form a complete loop emanating from the interface IC 101,running through each of LED driver ICs 81A-81H (collectively referred toas LED driver IC 81) and back to itself. Shifting data out of the SO pinof interface IC 101 concurrently returns an equal length bit string backinto the SI pin of interface IC 101.

SLI circuit 123 generates the SLI bus serial clock SCK signal asrequired. Because the LED driver ICs 81 have no chip addresses, thenumber of bits clocked through the SLI bus 113 must be properlycorrelated to the number of devices being driven. The number of devicesbeing driven, and hence the number of bits clocked through the SLI bus113, may be adjusted through software programming the data exchange inSPI bus interface 122, or by a hardware modification to interface IC101. In this manner, the number of channels within system 100 can bevaried flexibly to match the size of the display. The number of bitsshifted through the SLI bus 113, i.e. broadcasted on the bus 113,depends on the SLI bus protocol employed and the total number of bits inthe SLI bus shift registers 90A-90H. For example, the “fat” SLI busprotocol requires 72 to 88 bits per dual-channel LED driver, while theprefix multiplexed SLI bus is substantially smaller, e.g. a fixed 32bits per LED driver IC, regardless of the number of channels integratedinto each driver IC.

When using a hardware interface IC 101 to control SLI bus communication,modifying the registers in SLI bus circuit 123 to shift out fewer ormore bits requires a modification in the manufacturing or design ofinterface IC 101. An alternative approach involves replacing interfaceIC 101 with a programmable interface IC using software to adjust thedriver for accommodating fewer or more LED driver ICs in the daisychain.

The current sense feedback (CSFB) signal delivered to SMPS 108 isgenerated by an analog daisy chain, with the CSFB1 input pin on LEDdriver IC 81H tied via line 112I to Vcc, the CSFBO output pin of LEDdriver IC 112H connected via line 113H to the CSFB1 input pin of LEDdriver 81G, and so on, with each driver IC comprising one CSFB1 inputpin and one CSFBO output pin. Lastly, line 112B connects the CSFBOoutput pin of LED driver IC 81B to the CSFB1 input pin of LED driver IC81A, which in turn has its CSFBO output pin connected through line 112Ato the CSFB1 input pin of interface IC 101. The CSFB signal drops involtage whenever it passes through a driver IC driving an LED stringthat has a higher forward-voltage drop Vf than the preceding strings.

As a daisy chain, there is no one common line having one specificvoltage, but rather the CSFB voltage cascades from the first to the lastLED driver IC in the chain, the CSFB voltage on the last line 112Arepresenting the LED string with the highest Vf in the entire LED array.An operational transconductance amplifier OTA 127 converts the finalCSFB signal on line 112A into the current sense feedback (CSFB) signalat the ICSFB pin of interface IC 101, which via line 111 is delivered toSMPS 108. In response to the CSFB signal, SMPS 108 drives +V_(LED)voltage on supply rail 109 to the optimum voltage for flicker freelighting without excess power dissipation.

In system 100, only a single value of CSFB signal 11 is generated byinterface IC 101 and the CSFB daisy chain 112A-112I to drive SMPS 108.In systems where more than one +V_(LED) supply voltage is required, e.g.larger higher current backlit displays, or displays with RGBbacklighting, more than one interface IC 101 is required to power morethan one SMPS. For example, by repeating the entire system 100 for LEDsof different colors. i.e. one system for red LEDs, one for blue LEDs,and a third for green LEDs, the present architecture can be extended tomultiple SMPS solutions, albeit at a relative high cost. As such thethree instantiations of interface IC 101 would collectively communicateto a common backlight μC and scalar IC via a shared SPI bus, but wouldotherwise operate independently. Unfortunately such an approach alsotriplicates the number of interconnecting wires, greatly complicatingPCB design.

FIG. 5 is a simplified block diagram of system 100, illustrating thesignificantly reduced build-of-materials using intelligent LED driverswith SLI serial bus control and eliminating high pin-count packageinterface IC. As shown, sixteen strings of LEDs 83A-83Q are driven byonly eight small LED driver ICs 81A-81H, all controlled by interface IC101-SLI bus lines 113A-113I in response to host μC 7 and scalar IC 8.Compared to FIG. 1 comprising 32 discrete MOSFETs and a 72-pin interfaceIC, the system cost is greatly reduced by the new architecture. Withsignificantly fewer components the system reliability is also enhanced.

System 100 is also easy to deploy, since the SLI bus protocol is usedonly between interface IC 101 and the satellite LED drivers 81A-81H.Communication between the μC 7 and the interface IC 101 or the scalar IC8 still use the more complex higher-overhead SPI bus to communicate. Insome systems, the interface IC 101, microcontroller 7 and theintermediate SPI bus interface can be eliminated, and insteadalgorithmic control can be moved into the scalar IC 8 to facilitate afully scalable system entirely under software control.

As shown, only two analog signals are present in system 100, a commonVref on one of lines 107, and the daisy-chained CSFB signal on lines113A-113I, with either the analog feedback voltage CSFB or optionallythe analog feedback current ICSFB signal controlling the +V_(LED) outputof SMPS 108. In cases where SMPS 108 requires an analog current ratherthan a voltage for its feedback input, interface IC 101 is required toconvert the analog feedback voltage at its CSFB1 pin into an analogfeedback current ICSFB signal on line 111. An operationaltransconductance amplifier or OTA—a specialized precision analog circuitintegrated within mixed signal interface IC 101, performs this function.With few analog signals and no discrete DMOSFETs with high impedanceinputs, system 100 is relatively immune to noise.

Employing a single CSFB signal supplied via interface IC 101 derivedfrom the CSFB signal on lines 112A-112I to control SMPS 108 in system100, interface IC 101 is limited to operate in conjunction with a singleSMPS 108. With only a single power supply and feedback signal, themaximum power of the backlight module is limited to the power handlingcapability of SMPS 108. At high power levels, it becomes desirable to“split” the power supply up into multiple supplies in order to maintainhigher converter efficiency and cooler operation. In the configurationshown, it is not possible to provide multiple ICSFB signals withoutincreasing the number of interface ICs 101 to equal the number ofswitch-mode power supplies used.

Moreover, limited to a single +V_(LED) supply, the statistical range involtage mismatch of the LED strings increases with the number ofstrings, leading to higher power dissipation in the LED driver ICs81A-81H, higher delivered power required from SMPS 108, greater heating,and lower overall backlight system efficiency.

The single CSFB signal in system 100 also prevents its application inRGB backlight modules unless the entire system is triplicated, onesystem for driving strings of red LEDs, another for driving strings ofgreen LEDs, and a third system for blue LEDs.

Even with single CSFB line 11 to SMPS 108, every LED driver IC 81A-81Hstill must dedicate two of its sixteen pins to the analog CSFB1 andCSFBO signals, reducing the number of package pins otherwise availablefor increasing the number of LED driver channels, for incorporating newfeatures, or available to use for lowering the package's thermalresistance.

What is needed is a means by which to support multiple CSFB signals asneeded and beneficially to eliminate the two pins dedicated to the CSFBfunction on every LED driver IC.

LED Backlighting System with SLI Bus Embedded CSFB

To reiterate, system 100 shown in FIG. 5 illustrates that the currentsense feedback signals on lines 112A-112I connect LED driver ICs 81 tointerface IC 101 and ultimately through line 111 to SMPS 108. Thefunction of current sense feedback, or CSFB, is to measure the voltageacross every LED string 83 determine which string has the highestforward-voltage, and to control the +V_(LED) output one supply rail 109powering all the LED strings to insure that +V_(LED) is sufficient forevery string to operate at a specified and constant level of current.

One disadvantage of this approach is the analog CSFB signal requires twopins on every LED driver IC, which on a 16 pin package wastes one-eighthof the pins—pins that could be devoted to improving thermal resistance,adding functionality, or increasing the number of channels in thedriver. Another disadvantage of using the analog CSFB signal is there isno convenient means to facilitate supporting several CSFB signals forRGB and multiple SMPS backlight systems.

Since the signal on the SLI bus 113 also interconnects the same LEDdriver ICs 81 to interface IC 101, the CSFB signal can be embeddeddigitally in the SLI bus signal, eliminating the need for the analogCSFB signals on lines 112A-112H. The benefits of doing this aredisclosed herein.

FIG. 6 illustrates LED backlight system 170 with embedded CSFB. Comparedto system 100, described previously, the only components requiringchange to embed the CSFB signal into the SLI bus are the LED driver ICs174H-174H (sometimes referred to herein as driver ICs 174) and theinterface IC 171. As such, LED driver ICs 174 include ananalog-to-digital converter to convert the voltage feedback into adigital equivalent and to embed the information within the SLI bus datastream 161. This embedded CSFB signal within the digital SLI busprotocol is subsequently converted back into an analog signal by a DACor digital-to-analog converter contained within interface IC 171. Assuch, the CSFB function embedded within the SLI bus protocol andinterface beneficially eliminates the need for analog CSFB signals anddedicated package pins.

As described, the embedded CSFB function is implemented to control asingle SMPS and +V_(LED) supply rail. The inventive embedded CSFB methodcan readily be modified to control multiple power supplies for higherpower backlight systems or for RGB backlighting applications. Thisalternative embodiment of the invention is described later in thisapplication.

It should also be noted that in other embodiments of this invention,interface IC 171 may be eliminated and its functionality redistributedinto other components within the system. For example, the digitalfunctions of dimming, phase control, dot correction, and faultmanagement can be performed within μC 7 or within scalar IC 8 while theanalog Vref may be generated within SMPS 108, added into μC 7, orprovided by a small discrete IC. Likewise the conversion of the digitalrepresentation of the CSFB signal on the SLI bus 161 to an analog CSFBfeedback signal on line 160 can be integrated into SMPS 108, added intoμC 7 or provided by a small discrete IC, possibly also integrating anoperational transconductance amplifier and Vref within the same smallIC, e.g. within an 8-pin package.

To embed the CSFB signal into the SLI bus 161, different SLI busprotocols may be employed. One such protocol, the so-called “fat” SLIbus protocol and hardware involves a relatively long digital wordcontaining all of the parametric information for every channel and everyfunction in every SLI bus transmission. A second protocol, hereinreferred to as a “prefix-multiplexed” SLI bus reduces the size of theSLI bus command to a fixed length, e.g. 32 bits, and facilitatesupdating only those parameters that change without the need torebroadcast all the parametric data for every channel and function eachtime a specific update is required. The two SLI bus protocols and theimplementation of SLI bus embedded CSFB functionality in each version isdescribed in the following section.

Implementing Embedded CSFB into Fat SLI Bus Protocol & Interface

One implementation of an LED driver with an SLI bus-embedded CSFBfunction is shown in FIG. 7A. Included in an LED driver IC 200 are anSLI bus shift register 201, containing shift registers 220A, 221A, 222A,220B, 221B, 222B, 223, 224 and 225, a digital control and timing (DC&T)circuit 202 and an analog control and sensing (AC&S) circuit 203. Theexample shown is a dual-channel driver, but other number of channels maybe implemented in a similar fashion.

LED driver IC 200 is mixed signal, combining both digital and analogsignals, including digital SLI shift register 201 connected to a digitalDC&T circuit 202 by several parallel data busses, typically 12 bitswide, and also connected to an analog AC&S circuit 203 by a variety aparallel data busses ranging from 4 bits to 12 bits wide.

The outputs of DC&T circuit 202, digitally toggle I-Precise gate drivercircuits 206A and 206B and current sink DMOSFETs 205A and 205B on andoff with precise timing synchronized by the Vsync and GSK grey scaleclock signals. The current sink MOSFETs 205A and 205B control thecurrents I_(LEDA) and I_(LEDB) in two strings of LEDs, not shown, inresponse to analog signals from AC&S circuit 203 which in turn controlthe gate drive signals output by I-Precise gate driver circuits 206A and206B. The gate drive signals are analog, using an amplifier withfeedback to insure the conducting current in each of current sinkMOSFETs 205A and 205B is a fixed multiple of reference current Iref,also supplied by AC&T circuit 203.

While LED driver IC system 200 includes only current sink MOSFETs 205Aand 205B, the circuit is compatible with either the cascode clamped LEDdriver output as shown in FIG. 3A or the high voltage current sinkversion as exemplified in FIG. 3B and FIG. 3C. To implement the cascodeclamped version, two high-voltage N-channel DMOSFETs are connected inseries with current sink DMOSFETs 205A and 205B, with their sourceterminals tied to the drain terminals of the current sink DMOSFETs, andwith their drains tied to the anode of the respective LED strings beingdriven.

Both the drain voltage and the source voltage of current sink DMOSFETs205A and 205B are used to monitor the status of the LED strings by LEDdetect circuit 215, specifically the source voltages are used to detectopen LED strings while the drain voltages are used to detect shortedLEDs. The fault set latch 224 may be used to program the voltage levelsused to detect a shorted LED.

The drain voltages of current sink DMOSFETs 205A and 205B are also usedby a CSFB circuit 218A to determine the channel with the highest LEDvoltage drop, i.e. the DMOSFET with the lowest drain voltage. CSFBcircuit 218A outputs to an analog-to-digital (A/D) converter 218B avoltage equal to the lower of the drain voltages of current sinkDMOSFETs 205A and 205B, and A/D converter 2188 converts this lower drainvoltage into its equivalent digital value, storing it in a CSFB shiftregister 223 in SLI bus shift register 201. While the CSFB voltage canbe continuously updated, this is normally unnecessary to control therelatively low bandwidth SMPS powering the LED strings. One sample perVsync pulse in many cases may suffice.

In operation, data is clocked into SI shift register 201 through theserial input pin SI at a clock rate SCK. This includes shifting datainto 12 bit data registers 220A and 2208 for PWM on-time data forchannel A and channel B, 12 bit data registers 221A and 221B for phasedelay data for channel A and channel B, 12 bit data registers 222A and222B for “dot” current data for channel A and channel B, along with 12bits for Fault information comprising 8 bit register 224 for Fltsettings and 4 bit register 225 for Flt status. In addition, for driverswith embedded CSFB, SLI bus shift register 201 also includes a 4-bitregister 223 containing a word output by A/D converter 218A thatrepresents the CSFB voltage output of LED driver IC 200. Data withinthese registers are clocked out of the SO pin as new data is clocked in.Suspending the SCK signal holds data statically within the shiftregisters. The terms “channel A” and “channel B” are arbitrary and areonly used to identify the outputs and their corresponding data in theSLI data stream.

Upon receiving a Vsync pulse, data from PWM A register 220A is loadedinto D latch 211A and data from Phase A register 221A is loaded into Φlatch 212A of Latch & Counter A circuit 210A. At the same time, datafrom PWM B register 220B is loaded into D latch 211B and data from PhaseB register 221B is loaded into Φ latch 212B of Latch & Counter B circuit210B. Upon receiving subsequent clock signals on GSC grey scale clock,both counters count the number of pulses in their Φ latches 212A and212B and thereafter enable current flow in I-Precise gate drivercircuits 206A and 206B, respectively, illuminating the LED stringconnected to that specific channel. The channels remain enabled andconducting for the duration of the number of pulses stored in D latches220A and 220B. Thereafter, the outputs are toggled off and wait for thenext Vsync pulse to repeat the process. DC&T circuit 202 thereforesynthesizes two PWM pulses in accordance with the SLI bus data.

Also synchronized to the Vsync pulse, the data stored in Dot A and Dot Blatches 222A and 22B is copied into D/A converters 213A and 213B,setting the current in DMOSFETs 205A and 205B. As shown, the D/Aconverters 213A and 213B are discrete circuits providing a precisefraction of ref to set the currents in the LED strings. Alternatively,in a preferred embodiment, I-Precise gate driver circuits 213A and 213Bincorporate an adjustable current mirror using binary weighting and areable to set the fraction of the maximum current desired. The referencecurrent Iref, that represents the maximum channel current, is set byRset resistor 204 and the Vref input in bias circuit 217.

Fault detection includes an LED detect circuit 215 compares the sourceand drain voltages of current sink MOSFETs 205A and 205B against thevalues stored in Fault latch 214, which are copied from Flt Set register224 at each Vsync pulse. Temperature detect circuit 216 monitors thetemperature of the LED driver IC. Any fault immediately triggers opendrain MOSFET 219 to turn on and pull the FLT line low, generating aninterrupt. The fault information is also written from Fault latch 214into Flt Status register 225 on the following Vsync pulse.

For LED drivers with embedded CSFB, the digital data representing theCSFB value for LED driver IC 200 is copied from the output of A/Dconverter 218B to CSFB register 223 in SLI bus shift register 201,synchronized to the Vsync pulse. While it is possible to refresh theCSFB data more often than once per Vsync pulse, using the fat SLI busprotocol there is no specific convenient timing pulse to instruct thedata to be copied from A/D converter 218B into CSFB register 223.Without an additional dedicated control pin, the CSFB write operationmust be performed using a timer, but since the GSC and SCK clock signalsmay start and stop in normal operation there is no simple way to executeoversampling of the CSFB value.

Implementation of the dot function and digital-to-analog conversion isfurther detailed in the above-referenced application Ser. No.13/346,625, to Williams et al., entitled “Low Cost LED Driver withIntegral Dimming Capability”. That application also includes detailedcircuit implementation examples of fault and LED detect circuits 214 and215, reference current source 217, and current sense feedback (CSFB)circuit 218. Accordingly, the details of those components are notrepeated here.

In the manner described, a serial data bus is used to control themagnitude, timing, and duration of the currents in a number of LEDstrings, as well as to control the detection of and report theoccurrence of fault conditions in the LED strings, and control the+V_(LED) supply voltage using embedded CSFB information. The SLIprotocol is flexible, requiring only that the data sent through theserial bus match the hardware being controlled, specifically that thenumber of bits sent to each LED driver IC matches the bits required bythat LED driver IC (normally each LED driver IC in a given LED drivesystem requires the same number of bits), and that the total number ofbits sent during each Vsync period equals the number of bits required byeach LED driver IC times the number of LED driver ICs.

For example, in FIG. 7A the fat SLI bus protocol including dotcorrection, fault setting and fault reporting, and one channel of CSFBinformation comprises 88 bits per dual-channel driver IC, i.e. 44 bitsper channel or LED string. If eight dual-channel driver ICs controlling16 strings of LEDs are connected by a single SLI bus loop, the totalnumber of bits shifted out of the interface IC and through the SLI busduring each Vsync period is equal to 8 times 88, or 704 bits, less thana kilo-bit. If the SLI bus is clocked at 10 MHz, the entire data streamcan be clocked through every driver IC and to every channel within 70.4microseconds or 4.4 microseconds per channel.

While the serial data bus communicates at “electronic” data rates, i.e.using MHz clocks and Mbits-per-second data rates, the Vsync, or “frame,”rate used to control changing the image on the LCD display panel occursat a much slower pace because the human eye cannot perceive changingimages at anywhere near electronic data rates. While most people areunaware of flicker at 60 Hz frame rates, i.e. sixty image frames persecond, in A versus B comparisons, to many people 120 Hz TVs imagesappear more “clear” than 60 Hz TVs, but only using direct comparisons.At even higher Vsync rates, e.g. 240 Hz and up, only “garners” and videodisplay “experts” claim to see any improvement, mostly manifest asreduced motion blur. It is this large differential between electronicdata rates and the relatively slow video frame rate that makes serialbus communication to the backlight drivers possible.

For example, at 60 Hz, the each Vsync period consumes 16.7 milliseconds,orders-of-magnitude longer than the time needed to send all the data toall the driver-ICs. Even in the most advanced TVs running with an 8×scan rate and in 3D mode at 960 Hz, each Vsync period consumes 1.04milliseconds, meaning up to 236 channels can be controlled in real time.This number of channels greatly exceeds the LED driver systemrequirements for even the largest HDTVs.

The 88-bit per dual-channel “fat” protocol shown in shift register 201of FIG. 7A enables the interface IC to write or read all the data inevery register of every channel once during every Vsync period. The term“fat” refers to the content of the digital word used to control eachchannel. The fat protocol requires that data for every variable andregister in a given LED driver IC be included in each packet of datatransmitted from the interface IC to a that LED driver IC, even ifnothing changed from the prior data packet for that LED driver IC.

If a reduced data protocol is used, i.e. a protocol requiring fewer bitsper channel, sending data to every channel takes even less time. Sincethe fat protocol has no timing limitations because of the relativelyslow Vsync refresh rate, there is no data rate benefit. Using fewer bitsin the serial communication protocol does however reduce the size of thedigital shift registers and data latches in the LED driver ICs, reducingthe area of the LED driver IC and lowering the overall system cost.

For example, an alternative dual-channel data protocol for the SLI buswith embedded CSFB using 64 bits rather than the 88-bit data set shownfor LED driver IC 200 in FIG. 7A is also possible. Such a data set couldcomprise 12 bits for PWM brightness duty factor, 12 bits for phasedelay, 8 bits for fault setting, and 4 bits for fault status, and 4 bitsfor one channel of CSFB data, thereby excluding the 12-bit Dotcorrection data. As such, individual channel current setting andbrightness calibration of each LED string is not available in thisimplementation.

In LCD panel manufacturing, many manufacturers believe electronicallycalibrating a display for uniform brightness is too expensive and istherefore not commercially practical. Global display brightness canstill be calibrated by adjusting the value of a panel's current setresistors, such as set resistor 204 shown in LED driver IC 200, butbacklight uniformity in backlight brightness cannot be controlledthrough the microcontroller or interface IC. Instead, panelmanufacturers manually “sort” their LED supply into bins of LEDs havingsimilar brightness and color temperature.

It should be noted that removing Dot data from the SLI bus protocol doesnot prevent overall display brightness control or calibration. Adjustingthe system's global reference voltage Vref can still perform globaldimming and global current control. For example, in LED driver IC 200,adjusting the value of Vref affects the reference current Iref producedby Iref generator 217. If the Vref voltage is shared by all the LEDdriver ICs, adjusting this voltage will uniformly affect every driver ICand the panel's overall brightness independent of the PWM dimmingcontrol.

Embedding CSFB functionality into the SLI bus is not limited to the fatSLI bus protocol. To the contrary, greater flexibility and higher CSFBfeedback sample rates are more conveniently implemented using the prefixmultiplexed SLI bus protocol and interface.

As such, the limitations and disadvantages of sending long digital wordsor instructions over a serial bus can be circumvented through the use ofa “register address” or “prefix” added into the serial lightinginterface bus protocol and embedded in every SLI bus communication. Whencombined with circuitry to decode and multiplex the SLI bus data, theembedded prefix information enables data to be routed only to specifictargeted functional latches.

Implementing Embedded CSFB in Prefix Multiplexed SLI Bus Protocol andInterface

By sending data specifically only to latches requiring updates, the“prefix multiplexed” or “slim” SLI bus architecture avoids the need forrepeatedly and unnecessarily resending digital data, especiallyresending redundant data that remains constant or changes infrequently.In operation, after an initial setup, only latches that are changing arerewritten.

Registers containing fixed data are written only once when the system isfirst initialized, and thereafter do not require subsequentcommunication through the SLI bus from the interface IC. Because onlythe latches that are changing are updated, the amount of data sentacross the SLI bus is greatly reduced. This inventive method offersseveral distinct advantages over the fat SLI bus method, namely:

-   -   the number of bits required to integrate a SLI bus shift        register is greatly reduced, saving die area and lowering cost,        especially in smaller (e.g. two channel) LED driver ICs    -   the effective bandwidth of the SLI bus at any given clock rate        is increased because redundant data is not being repeatedly sent    -   the SLI bus protocol can be standardized with fixed word lengths        and functions without losing versatility

An example of a prefix-multiplexed SLI bus is shown in the embodiment ofLED driver IC 230 shown in the schematic circuit diagram of FIG. 7B. Inaddition to the alternative LED driver IC 230, FIG. 7B also shows an SLIbus shift register 231 containing a 16-bit prefix register 232 and a16-bit data register 233, and a prefix decoder and multiplexer (mux)circuit 234. The data in data register 231 is routed to D latches 211Aand 211B and Φ latches 212A and 212B in Latch & Counter A 210A and Latch& Counter B 210B, respectively, to one of D/A converters 213A and 213Bin digital control and timing (DC&T) circuit 202, or to a fault latchcircuit 214 in analog control and sensing (AC&S) circuit 203. These datatransfers are made through prefix decoder and multiplexer circuit 234according to routing directions contained in prefix register 232. Thusprefix decoder & multiplexer circuit 234 decodes the 16-bit word storedin prefix register 232 and multiplexes the 16-bit data 223 stored indata register 233 into the appropriate D, Φ, or Dot latch DC&T circuit202 or AC&S circuit 203.

In the case of fault latch circuit 214, multiplexer 234 operatesbidirectionally, allowing the data stored in data register 233 to bewritten into fault latch circuit 214 or, conversely, allowing the datastored in fault latch circuit 214 to be written into data register 223.Similarly, depending on the prefix code stored in prefix register 232,the CSFB data contained within A/D converter 218B is directed bymultiplexer 234 to be written into data register 233.

While in the case of the fat SLI bus protocol, data is copied betweenthe functional latches and the SLI bus registers in synchronism with theVsync pulse, in the prefix-multiplexed SLI bus, some functions need notbe synchronized to the Vsync pulse, especially in the case of readingback fault information from fault latch 214 and CSFB data from A/Dconverter 218B. Instead, the data can be “pulled” from the LED driver ICinto the SLI bus and examined by the interface IC as needed, even at adata rate higher than once per Vsync pulse.

In a preferred embodiment the prefix multiplexed SLI bus protocolcomprises a 32-bit word, i.e. 4 bytes in length, offering awell-balanced compromise between the flexibility of addressing a largenumber of functional latches and maintaining a short word length andsmall SLI bus shift register size. In the example shown, SLI bus prefixregister 232 is 16-bits in length and SLI bus data register 233 is also16-bits in length, facilitating a variable with up to 65,536combinations to be uniquely written or read from one of 65,536 differentfunctional latches.

At 32-bits, the prefix multiplexed SLI bus protocol is designed forflexibility and expandability. Despite facilitating a large number ofcombinations, not all of the data stored in the SLI bus registers needbe used. If fewer latches and channels are needed only a few bits of theprefix need be decoded to address the requisite number of functionallatches. Likewise, if less than 16 bits of precision is demanded, asmaller number of bits may be used in the data register in the SLI busand multiplexed to the target functional latch. For example, if the datacontained in SLI bus data register 233 represents the PWM brightnessduty factor, 12 bits of data may be multiplexed and loaded into D latch211A while if the data contained in SLI bus data register 233 representsthe LED current “Dot” setting, only 8-bits may be required by Dot latchwithin D/A converter 213A. The CSFB data read from A/D converter andlatch 218B and written into SLI bus data register 233 may constituteonly a 4-bit word.

So in the prefix-multiplexed SLI bus, data is repeatedly written by theinterface IC into the SLI bus shift register 231 and then multiplexedinto one of several functional latches 211-214 one word at a time, insequential fashion. Likewise data is copied from latches 214 and 218Bwhenever requested by the interface IC, and shifted through the shiftregisters in the daisy chain and back to the interface IC in sequentialfashion. In the LED driver IC 200 shown in FIG. 7B, one SLI bus dataregister 233 fans out into seven different functional latches and readsdata back from two functional latches.

The prefix multiplexed SLI bus 230 is in sharp contrast to the fat SLIbus shown in FIG. 7A, wherein each register in SLI bus shift register201 has a one-to-one correspondence to a functional latch in the LEDdriver IC, e.g. SLI bus PWM A register 220A corresponds to D latch 211A,SLI bus Phase A register 221A corresponds to Φ latch 212A, and so on.This on-to-one correspondence makes scaling the fat SLI bus architectureto LED driver ICs having more channels problematic and costly.

The fan out capability of the prefix-multiplexed SLI bus thereforeoffers a more versatile lower cost approach to implement a multichannelLED drive than the fat SLI bus protocol. For this and other reasons tobe considered later in this disclosure, the inventive prefix-multiplexedSLI bus represents an improved serial lighting interface bus protocol,architecture, and physical interface.

Prefix decoder and multiplexer 234 can be realized in a variety of ways,as described in the above-referenced application Ser. No. 13/346,647, toWilliams et al., entitled “Low Cost LED Driver with Improved SerialBus”. One implementation is shown in the block diagram FIG. 8, whereinthe 16-bit prefix register in SLI bus shift register 231 is subdividedinto two 8-bit registers, a channel register 232C and a functionregister 232F. The data register 233 remains unchanged. As shown, prefixdecoder 251 has two output lines comprising a channel select output line254 to select which LED channel 255 is being controlled, and a functionselect output line 252 to control which functional latch is beinginterrogated, i.e. the functional latch being written to or read from.

In the example shown, prefix decoder 251 selects one of the manychannels 255 with a channel select signal on line 254, then chooses thefunction to be controlled with function select signal on line 252. Tochange operation of a function 256, multiplexer 253 then writes datafrom data register 233 into a preload latch 258. The data is preservedin preload latch 258 until a Vsync pulse occurs, at which time the datais copied from preload latch 258 into an active latch 257, therebychanging the operating conditions of analog or digital function 258,e.g. D, Φ, Dot, etc. The data in active latch 257 remains unchanged tillthe next Vsync pulse occurs.

Control function 259 may be changed in a similar fashion by writing datainto preload latch 261, copying the data into active latch 260synchronized to the Vsync pulse, and thereby changing the operatingconditions of the selected channel. Alternatively, data in latch 260 maybe written from control function 259 and loaded, i.e. sampled, intopreload latch 261 at regular intervals. The data contained in preloadlatch 261 is then copied into the data register 233 of SLI bus shiftregister 231 with decoder 251 selecting the corresponding channel andfunction.

In this manner any number of channels within a LED driver IC, i.e. anynumber of LED strings, can be controlled independently in real timefacilitating precise adjustment of each control function 256, 259 andothers through a shared SLI bus shift register 231 without the need fora large shift register or long digital words.

As shown in FIG. 8, the same SLI bus shift register 231 andprefix-multiplexed SLI bus protocol can be employed to embed the CSFBfunction. The CSFB signal 262, the digital output of an A/D converter issampled at regular intervals and written into sample latch 263. In someembodiments the A/D converter and sample latch are part of the sameunit, as with the A/D converter 218B shown in FIG. 7B. In this mannerthe most current value of the CSFB for a given channel and LED driver ICis ever-present in sample latch 263. The data contained in sample latch263 is then copied into the data register 233 of SLI bus shift register231 whenever decoder 251 selects the corresponding channel and selectsthe CSFB function. Like any other function, the CSFB data is selected bythe appropriate and corresponding prefix code. The prefix codes in Table1 are included as an example for decoding:

TABLE 1 Channel Select Decode Hex Bits Decoded Channel Selected 00 xxxxxx00 Channel A 01 xxxx xx01 Channel B 02 xxxx xx10 Channel C 03 xxxxxx11 Channel D Function Select Decode Hex Bits Decoded Function Selected00 xxxx xx00 PWM Brightness (D) 01 xxxx xx01 Phase Delay (Φ) 02 xxxxxx10 Dot Correction 03 xxxx xx11 Fault Status & Reporting 04 xxxx x100Fault Set 05 xxxx x101 Fault Clear 06 xxxx x110 PWM Invert 0E xxxx 1110Load CSFB into SLI Bus 0F xxxx 1111 Execute CSFB

After the data is written into the SLI bus shift register 231, the datamust be shifted into the interface IC by the corresponding number of SCKpulses. The number of SCK pulses required to shift the data entirelythrough the SLI bus is equal to the number of bits in each SLI bus shiftregister times the number of SLI bus shift registers in the SLI bus.Assuming the fixed-length 32 bit protocol shown in FIG. 8 and one SLIbus shift register per LED driver IC, then the total number of SCKpulses required to shift data from the driver IC farthest from theinterface IC in the daisy chain into to the CSFB1 input of the interfaceIC is equal to 32 times the number of LED driver ICs.

During the shifting, the prefix code is selected to prevent overwritingof the data in the SLI bus shift register 233. In a preferred embodimentthis protection can be accomplished by using a dedicated prefix functioncode 251, e.g. hexadecimal 0E, for loading the SLI bus data registers233 from their corresponding CSFB sample latches 263. On the subsequentbroadcast, as the data from the sample latches 263 is shifted throughthe SLI bus daisy chain and into the interface IC to update the feedbacksignal controlling the SMPS, a different prefix code, e.g. hexadecimal0F, is used to prevent any reading or writing into or from the SLI busshift register 233 (this step is referred to as “Execute CSFB” in Table1 above).

Embedding CSFB functionality within the SLI bus is further explained bythe schematic block diagram of the LED drive system 270 shown in FIG. 9.Shown are LED driver ICs 272A and 272H, an interface IC 273, LED strings274A, 274B, 274P and 274Q, and a switch mode power supply (SMPS) 293.Relating to embedded CSFB operation, LED driver IC 272A contains currentsink MOSFETs 275A and 275B driving LED strings 274A and 274B,respectively, a CSFB circuit 291A, an analog to digital (A/D) converter275A, a sample latch 277A, along with SLI bus communication using an SLIbus shift register 282A comprising a prefix register 281A and a dataregister 280A, a decoder 279A and its associated multiplexer 278A. Otherfunctions within LED driver IC 272A such as PWM dimming control, dotcorrection, and fault detection are excluded for clarity's sake.

Similarly, LED driver IC 272H contains current sink MOSFETs 275P and275Q driving LED strings 274P and 274Q, respectively, a CSFB circuit291H, an A/D converter 275H, a sample latch 277H, along with SLI buscommunication using SLI bus shift register 282H comprising a prefixregister 281H and a data register 280H, a decoder 279H and itsassociated multiplexer 278H. Other LED driver ICs 272B-272G, not shown,are of identical construction. Interface IC 273 connects digitally toLED driver ICs 272A-272H through SLI bus 294, wired in daisy chainfashion with each SLI bus output SO wired to the SLI bus input SI of thenext IC in the daisy chain. The last LED driver IC 272H in the chain hasits SO output connected to the SLI input of interface IC 273. The firstLED driver IC in the chain 272A has its input connected to the SO outputof the interface IC 273 (the output portion of interface IC 273 is notshown) or alternatively, any other source of SLI bus data used tocontrol the LED driver ICs.

Interface IC 273 comprises SLI bus shift register 283 comprising aprefix register 285 and a data register 284, a decoder 287 and itsassociated multiplexer 286, a digital magnitude comparator 288, aregister 289 for storing a digital DCSFB signal, which is the currentlowest CSFB sample obtained from LED driver ICs 272A-272H, adigital-to-analog (D/A) converter 290, and an operationaltransconductance amplifier 291, connected in the LED drive system 270shown in FIG. 9 to the feedback input of SMPS 293, using an ICSFBsignal. Interface IC 273 is capable of providing two analog outputs: thecurrent feedback signal ICSFB output by OTA 291, or alternatively avoltage feedback signal CSFBO output by D/A converter 290 used toconnect to SMPS modules requiring a voltage rather than a currentfeedback signal.

Both the ICSFB signal and the CSFBO signal represent the analogequivalent of the digital current sense feedback signal DCSFB. Thisdigital DCSFB signal, sampled at regular intervals, represents thelowest current sense (drain) voltage on current sink MOSFETs 275A-275Q,and it is used to detect the LED string 274A-274Q with the highestforward-voltage drop. The CSFB signal (whether in the form of ICSFB orCSFBO) in turn controls the voltage +V_(LED) of SMPS 293 on the supplyline 271 to produce a voltage +V_(LED) sufficient to power all the LEDstrings 274 including whichever one has the highest forward-voltagedrop.

In operation, the interface IC 273 clocks a prefix command into prefixregister 281 via the SLI bus daisy chain to each of the eight LED driverICs 272A-272H, instructing each driver IC's multiplexer 278 to copy thecurrent sample contents of sample latch 277 into the data register 280of its SLI bus shift register 282. As an example, in driver IC 272A aprefix command in prefix register 281A, interpreted by decoder 279Ainstructs multiplexer 278A to copy the current sample contents of samplelatch 277A into the data register 280A of SLI bus shift register 282A.The same process and procedure occurs in the other LED driver ICs.

Prior to or contemporaneous with data transfer from sample latch 277Ainto SLI bus data register 280A, CSFB circuit 291A measures the drainvoltages on current sink MOSFETs 275A and 275B, determines which MOSFEThas the lower drain voltage, and delivers that lower drain voltage toA/D converter 276A, which converts the voltage into its digitalequivalent. The result is temporarily stored in sample latch 277A. TheCSFB voltage may be sampled at the time the data is requested throughthe prefix code in prefix register 281A, or it may be sampled at regularintervals more frequently than the SLI bus communications. As a result,the data contained within sample latch 277A represents the most currentinformation regarding the lowest current sink voltage of the twochannels integrated within LED driver IC 272A.

Voltage sampling should occur at least once per Vsync period and mayoccur at a higher rate. Preferably, it should occur two to three timesper Vsync period to improve the accuracy and transient response of SMPS293. Sampling above three times per Vsync period offers diminishingreturns, and excessive sampling, e.g. ten times the Vsync period,occupies the interface IC performing unnecessary tasks. Once the CSFBdata provided by A/D converters 277A-277H of LED-drivers 272A-272H hasbeen loaded into their corresponding SLI bus data registers 280A-280H,the CSFB data must be clocked out of the data registers and intointerface IC 273. The number of SCK pulses required to complete this isequal to the number of LED driver ICs 272 in the daisy chain times thenumber of bits per SLI bus shift register 282 in the protocol, in thiscase 32 bits times 8 driver ICs, or 256 clock pulses.

During or after the shifting of the driver IC CSFB data into SLI bus 283within interface IC 273, interface IC 273 performs the task ofdetermining which CSFB value is lowest and then uses that data todeliver a CSFBO or ICSFB signal to the SMPS 293, which in turn uses thatsignal to set the voltage +V_(LED) on the supply rail 271. While thedata can be stored and analyzed at the conclusion of the SLI bus shiftregister data transfer, it may also be performed in real time. In oneembodiment of this invention, each CSFB value shifted into the SLI busdata register 284 of interface IC 273 is compared to the previous valueby magnitude comparator 288 and overwritten into register 289 only whenit represents a lower voltage than the data before it. After all theCSFB data from all the driver ICs has been compared within one SLIbroadcast cycle, the digital data DCSFB within register 289 representsthe lowest CSFB value in the system 270.

In another aspect of this invention, a dedicated prefix code in prefixregister 285, e.g. hexadecimal code “0F” (binary “00001111”) can be usedduring the SLI bus shifting operation to prevent overwriting of the CSFBdata by LED driver ICs 272 while the data is shifted through the SLI busdaisy chain. The same prefix code can be chosen to instruct interface IC273 to perform the sequential comparison of the incoming data duringshifting, i.e. during sequential SCK pulses, to determine the lowestCSFB signal in the data stream. In this comparison operation, decoder287 directs multiplexer 286 to the input of digital magnitude comparator288. This circuit compares the data in SLI bus register 284 against thatin register 289 and overwrites register 289 only if the new data islower. The process is repeated until the CSFB data from every LED driverIC has been shifted into interface IC 273. In other words, in oneembodiment of this invention, a special prefix code can be assigned thatduring shifting never allows the multiplexer to overwrite the CSFB dataalready present in the SLI bus data stream.

Interface IC 273 then converts this digital representation of the lowestCSFB voltage into an analog voltage CSFBO or analog current ICSFB,collectively as feedback signals 292, which control the SMPS 293. Thenature of the analog feedback signal depends on the type of feedbackrequired by SMPS 293. If an analog voltage is required, the CSFBOvoltage output of D/A converter 290 can be used, with or without abuffer, to directly drive the SMPS 293. If a current feedback signal isrequired, then operational amplifier OTA 291 is used to convert theCSFBO voltage signal into the current signal ICSFB.

Regardless of whether the feedback to SMPS 293 comprises a current orvoltage, in closed loop operation the output voltage of the D/Aconverter 290 reacts to the digital CSFB signal DCSFB provided bydigital magnitude comparator 288, i.e. it becomes a dynamic function ofthe DCSFB signal. In this manner, real time feedback can be provided toSMPS 293 digitally-SLI bus 294, facilitating control of the LED powersupply output voltage +V_(LED) to guarantee adequate voltage for properillumination of LED strings 274A-274Q at requisite levels of LEDcurrent.

LED Driver IC with SLI Bus Embedded CSFB

An LED drive system 300 with SLI bus communication and embedded CSFBcontrol in accordance with this invention is shown in FIG. 10A. Similarto LED driver I 51 shown in FIG. 3A, LED drive system 300 comprises adual channel driver IC 301 with integrated current sink DMOSFETs 55A and55B, cascode clamp DMOSFETs 57A and 57B with integral high-voltagediodes 58A and 58B, I-precise gate driver circuits 56A and 56B foraccurate current control, a digital control and timing (DC&T) circuit59, and an on-chip bias supply and regulator 62. Unlike the previouslydescribed driver IC 51, however, the analog control and sensing (AC&S)circuit 310, along with fat SLI bus shift register 311, have beenmodified to embed current-sense-feedback CSFB information within the SLIbus protocol. Since the SLI bus shift register is “fat,” it contains aCSFB register (equivalent to register 223 in FIG. 7A) that is dedicatedto receiving CSFB data from a sample latch within AC&S circuit 310(equivalent to latch 277 in FIG. 9).

As such, LED driver IC 301 provides complete control of two channels of250 mA LED drive with 150V blocking capability and ±2% absolute currentaccuracy, 12 bits of PWM brightness control, 12 bits of PWM phasecontrol, 8 bits of dot current control, fault detection for LED open andLED short conditions and over-temperature detection, all controlledthrough a high-speed SLI bus, and synchronized to other drivers by acommon Vsync and grey-scale clock (GSC) signal. While the specificexample shown illustrates cascode clamp DMOSFETs rated at 150V blockingcapability, the devices can be sized for operation from 100V to 300V asneeded. The device's current rating of 250 mA is set by the powerdissipation of the package and the mismatch in forward voltage in thetwo LED strings being driven.

AC&S circuit 310 within LED driver IC 301 also includes an analogcurrent sense feedback, or CSFB, signal monitoring the two current sinkDMOSFETs 55A and 55B, converted by an integrated analog-to-digital A/Dconverter into a digital version of the CSFB voltage, preferably 4 ormore bits in length. This digital CSFB signal, or DCSFB, represents thelowest current source voltage in driver IC 301, and hence the LED stringwith the highest forward drop. This signal is copied to the CSFBregister in SLI bus shift register 311 and is passed through the SLI busto the interface IC and ultimately back to the system SMPS, supplyingthe +V_(LED) supply rail.

Unlike previously described LED driver IC 51, which requires CSFBO andCSFB1 pins for the output and input CSFB signals, respectively, LEDdriver IC 301 embeds its CSFB data into the SLI bus data stream andrequires no extra pins to facilitate current sense feedback regardlessof the number of channels integrated in the driver. The CSFBO and CSFB1pins are therefore absent from the package that contains LED driver IC301. Accordingly, analog control and sensing circuit 310, along with fatSLI bus interface 311, have been modified to embedcurrent-sense-feedback CSFB information within the SLI bus protocol.

An alternative LED drive 315 with SLI bus communication and embeddedCSFB control in accordance with this invention is shown in FIG. 10B. Adual-channel LED driver IC 316 contains current sink DMOSFETs 72A and72B but omits cascode clamp MOSFETs. Instead, DMOSFETs 72A and 72Bcontain integral high-voltage diodes 73A and 72B designed to sustainhigh-voltages in the off condition. Typically such a design is mostapplicable for operation below 100V but can be extended to 150V ifrequired. As in LED driver 301, I-precise gate driver circuits 71A and71B facilitate accurate current control, controlled by an analog controland sensing circuit 320, and a digital control and timing circuit 74. Anon-chip bias supply and regulator 69 powers LED driver IC 316, in thiscase from Vcc, not from the 24V input as in LED driver IC 301. Asidefrom lacking cascode clamp DMOSFETs, LED driver IC 316 operatessimilarly to LED driver IC 301, controlled through its SLI bus shiftregister 325 including a digital CSFB signal embedded within the SLI businterface and protocol.

Unlike the previously described LED driver IC 66 (FIG. 3B), whichrequires CSFBO and CSFB1 pins for the output and input CSFB signals,respectively, LED driver IC 316 embeds its CSFB data into the SLI busdata stream and requires no extra pins to facilitate current sensefeedback regardless of the number of channels integrated in the driver.The CSFBO and CSFB1 pins are therefore absent from the package thatcontains LED driver IC 316. Accordingly, analog control and sensingcircuit 320, along with fat SLI bus interface 325, have been modified toembed current-sense-feedback CSFB information within the SLI busprotocol.

An LED drive 330 using prefix-multiplexed SLI bus communication withembedded CSFB made in accordance with this invention is shown in FIG.10C. A dual-channel LED driver IC 331 comprises integrated current sinkDMOSFETs 87A and 87B with integral high-voltage diodes 88A and 88B,I-precise gate driver circuits 86A and 86B for accurate current control,an analog control and sensing circuit 335, and a digital control andtiming circuit 89. An on-chip bias supply and regulator 84 powers the ICfrom a Vcc input.

Unlike the previously described LED driver IC 80 (FIG. 3C) whichrequires CSFBO and CSFB1 pins for the output and input CSFB signals,respectively, LED driver IC 331 embeds its CSFB data into the SLI busdata stream and requires no extra pins to facilitate current sensefeedback regardless of the number of channels integrated in the driver.The CSFBO and CSFB1 pins are therefore absent from the packagecontaining LED driver IC 80. Accordingly, analog control and sensingcircuit 335, along with prefix-multiplexed SLI bus interface 340, havebeen modified to embed current-sense-feedback CSFB information withinthe SLI bus protocol.

Otherwise, LED driver IC 331 provides complete control of two channelsof 250 mA LED drive with 150V blocking capability and ±2% absolutecurrent accuracy, 12 bits of PWM brightness control, 12 bits of PWMphase control, 8 bits of current control, fault detection for LED openand LED short conditions and over-temperature detection, all controlledthrough a high-speed SLI bus, and synchronized to other drivers by acommon Vsync and grey-scale clock (GSC) signal. While the specificexample shown illustrates current sink DMOSFETs rated at 150V blockingcapability, the devices can be sized for operation from 100V to 300V asneeded. The device's current rating of 250 mA is set by the powerdissipation of the package and the mismatch in forward voltage in thetwo LED strings being driven. Above a 100V rating, it is advantageous tointegrate high voltage cascode clamp DMOSFETs (not shown) in series withcurrent sink DMOSFETs 87A and 87B, whereby current sink MOSFETs 87A and87B do not require operation above the clamp voltage, i.e. above 12V.

LED Drive System and Interface with SLI Bus Embedded CSFB

System 350 in FIG. 11 illustrates the application of a distributedsystem for LED backlighting with local dimming including SLI busembedded current sense feedback made in accordance with this invention.The figure illustrates an interface IC 351 driving a series of LEDdrivers 316A-3161H with integral dimming and fault detection powered bya common SMPS 353. The diagram is similar to system 100 in FIG. 4 exceptthat the analog CSFB daisy chain has been completely eliminated andfunctionally replaced by a DCSFB signal embedded within the SLI busprotocol and physical interface. While the analog-to digital conversionis not explicitly shown in LED driver ICs 316A-316H, interface IC 351does illustrate the addition of D/A converter 365 needed to reconstructthe analog feedback signal from the SLI bus embedded DCSFB digital word.

Each of LED driver ICs 316A-316H may employ a prefix multiplexed SLI busprotocol, as shown in the LED driver IC 331 of FIG. 10C or mayalternatively employ a fat bus protocol as shown in the LED driver ICs301 and 316 of FIG. 10A and FIG. 10B. Each of driver ICs 316A-316H maylikewise incorporate high voltage current sink MOSFETs as shown in FIG.10B and FIG. 10C or alternatively may integrate cascode clamp MOSFETs toprotect the current sink MOSFETs, as shown in FIG. 10A. In the system350, LED driver ICs 316A-316H are illustrated without a multiplexer 92and a decoder 91, with the understanding that such devices can beincluded within driver ICs 316A-316H as needed, i.e. whenever a prefixmultiplexed SLI bus protocol is utilized. Five common signal lines 357,comprising three digital clock lines, one digital fault line, and oneanalog reference voltage line connect interface IC 351 to every driverIC. A timing and control circuit 363 generates the Vsync and GSC signalsin sync with data from a host μC (not shown) received through an SPI businterface 360. Timing and control circuit 363 also monitors the FLTinterrupt line to immediately detect a potential problem. A referencevoltage source 362 provides a reference voltage Vref to the systemglobally in order to insure good channel-to-channel current matching. Abias supply 361 powers interface IC 351 from a supply voltage VIN on afixed +24V supply rail 354 generated by SMPS 353. Bias circuit 361 alsogenerates the regulated supply voltage Vcc, preferably 5V, to power LEDdrivers 316A-316H. The Vcc supply is filtered by a capacitor 362.

In this example, each of LED drivers 316A-316H comprises two channelswhich include high-voltage current sink DMOSFETs 72A-72Q with integralHV diodes 73A-73Q, I-Precise gate driver circuits 71A-71Q, DC&T circuits74A-74H, AC&S circuits 320A-320H including current sense feedbackdetection and A/D conversion into digital DCSFB, and SLI bus shiftregisters 325A-325H. While the LED driver ICs 316A-316H shown in FIG. 11lack cascode clamp MOSFETs, the system 350 may also be constructed inthe manner of LED driver IC 300 shown in FIG. 10A, except that the 24VVIN supply, rather than Vcc, may be used to power the LED driver IC'sand bias the gates of the cascode clamp DMOSFETs.

Any of the three versions 10A, 10B or 10C can be plugged into the driverIC boxes 316 in FIG. 11.

An SLI bus 356 connecting the LED driver ICs 316A-316H comprises SLI busshift registers 325A-325H connected together in a daisy chain by SLI buslines 356A-3561, where the SO serial output of SLI circuit 364 withininterface IC 351 connects via SLI bus line 356A to the SI input of LEDdriver 316A, the SO output of LED driver 316A connects via SLI bus line356B to the SI input of LED driver 316B (not shown) and so on. SLI busline 356H connects to the SI input of the last LED driver 316H shown insystem 350. The SO output of LED driver 3161-H, in turn, connects viaSLI bus line 3561 to the SI input of SLI circuit 364 within interface IC351. In this manner SLI bus 356 forms a complete loop emanating from theinterface IC 351, running-every LED driver IC 316A-316H (sometimesreferred to collectively as LED driver ICs 316) and back to itself.Shifting data out of the SO pin of interface IC 350 concurrently returnsan equal length bit string back into the SI pin of interface IC 350.

SLI circuit 364 generates the SLI bus clock signal SCK as required.Because the LED driver ICs 316A-316H have no chip addresses, the numberof bits clocked through the SLI bus 356 is correlated to the number ofLED driver ICs being driven. The number bits clocked through the SLI bus356 may be adjusted by modifying the software controlling the dataexchange in SPI interface 360, or by a hardware modification tointerface IC 351. In this manner, the number of channels within system350 can be varied flexibly to match the size of the display. The numberof bits shifted through the SLI bus 356, i.e. broadcasted on the bus356, depends on the SLI bus protocol employed and the number of bits inthe SLI bus shift registers. For example, the fat SLI bus protocolrequires 72 to 88 bits per dual channel LED driver while the prefixmultiplexed SLI bus is substantially smaller, e.g. a fixed 32 bits inlength per LED driver IC regardless of the number of channels integratedinto each driver IC.

When a hardware controller within interface IC 351 is used to controlSLI bus communication, modifying the registers in SLI bus circuit 364 toshift out fewer or more bits requires a modification in themanufacturing or design of interface IC 351. An alternative approachinvolves replacing interface IC 351 with a programmable interface ICusing software to adjust the driver for accommodating fewer or more LEDdriver ICs in the daisy chain.

Current sense feedback to SMPS 353 comprises a digitalcurrent-sense-feedback or DCSFB signal embedded within SLI bus 356.Shifting data through the SLI bus 356 ultimately returns this embeddedDCSFB signal to SLI bus circuit 364 of interface IC 351. In the mannerdescribed previously, SLI bus circuit 364 in turn, outputs the a DCSFBword representing the lowest CSFB word in SLI bus 356 and D/A converter365 converts that DCSFB word into an analog CSFB feedback voltage.Operational transconductance amplifier 366 then converts the CSFBfeedback voltage into current feedback ICSFB signal on line 358 tocontrol the +V_(LED) output of SMPS 353. Alternatively, the CSFBfeedback voltage itself may be used as the feedback signal to controlSMPS 353. Digitally embedding the CSFB data contrasts system 350 withthe system 100 of FIG. 4, wherein connecting every the LED driver IC tothe interface IC through an analog daisy chain requires two dedicatedpins per driver IC.

In system 350, like in system 100, only a single value of CSFB feedbacksignal CSFBO or ICSFB is generated by interface IC 351. In applicationswhere more than one SMPS is required, e.g. in larger, higher currentbacklit displays or displays with RGB backlighting, the interface IC canbe modified to output more than one CSFB output voltage to controlmultiple SMPS units. The SLI bus data stream itself carries thenecessary information to independently control multiple LED supplyvoltage rails, but the interface IC has to be configured to separate thechannel information appropriately to take advantage of such a feature. Amulti-feedback, multi-output embodiment of this invention is describedbelow.

Analog and Digital Data Conversion

Referring again to FIG. 9, the voltages present across the current sinkMOSFETs 275 are dynamically measured and used to control the outputvoltage of SMPS 293. While the measured voltage and the feedback signalboth comprise analog signals, the SLI bus embedded CSFB method inaccordance with this invention comprises a digitally encoded feedbackpath from the LED driver ICs 272 to the interface IC 273. Such a systemrequires analog-to-digital conversion within the LED driver ICs 272 tosense the voltages across the current sink MOSFETs 275, anddigital-to-analog conversion within the interface IC 273 to generate thefeedback signal 292 for SMPS 293.

Changing analog signals into digital words and vice versa, relies ondata conversion. While designs of A/D and D/A converters are well knownto those skilled in the art, a large variety of converters exist andmust be selected to meet, but not substantially exceed, the performancerequirements of the digital CSFB function. Data converter designs thatrespond too slowly to load transients or suffer from instability andlong settling times can lead to flicker and inconsistent display imagesand in extreme cases may even damage electronic components in a display.Conversely, accurate high performance converters are generally too largeand too expensive for the TV market.

FIG. 12 illustrates circuitry for sensing and digital encoding performedwithin the LED driver ICs where the voltage on the drain connection ofMOSFETs 275A and 275B driving LED strings 274A and 274B is connected tothe positive inputs V_(inA)+ and V_(inB)+ of a CSFB circuit 275 whichcomprises an operational amplifier. With the output of the operationalamplifier connected to its negative input V_(inB)−, the operationalamplifier acts as a unity gain amplifier, or voltage-follower,amplifying the most negative of its positive inputs V_(inA)+ andV_(inB)+. The output of CSFB circuit 275 also feeds the input of an A/Dconverter 276. Note that the I-Precise gate driver circuits for currentsink MOSFETs 275A and 275B are not shown in FIG. 12 for the sake ofclarity.

In one embodiment of CSFB circuit 275, the operational amplifiercomprises a differential input with matching input P-channel MOSFETs401A, 401B and 401C and current source 403. A current mirror comprisingmatching N-channel MOSFETs 402A and 402B reflects the current innegative input P-channel MOSFET 401A. The current in N-channel MOSFET402B along with current in P-channel MOSFETs 401B and 401C is summed,driving a second amplifier stage comprising an N-channel MOSFET 404 andactive load 405 comprising a current source. Along with negativefeedback from its output to negative input MOSFET 401A, a compensationnetwork comprising capacitor 406 and resistor 407 is included to set thepole-zero response of the amplifier and maintain stability over the fullrange of operation.

In operation, the most negative input present on the amplifier'sV_(inA+) and V_(inB+) inputs turns on either P-channel MOSFET 401B or401C more than its parallel counterpart, and forces the output of theamplifier to the lower of the two drain voltages of MOSFETs 401B and401C. The output is then digitized by A/D converter 276 and loaded intothe SLI bus shift register when requested.

While CSFB circuit 275 in FIG. 12 is shown for a dual-channel LED driverIC, any number of channels can be integrated simply by adding positiveinputs connected to P-channel MOSFETs matched to MOSFETs 401B and 401C.For example, if a third positive input were connected to a P-channelMOSFET 401D, then CSFB circuit 275 would output the lowest of its threeinputs, either that of channel A, channel B or channel C. In this mannerCSFB circuit 275 detects and outputs the lowest drain voltage of anycurrent sink MOSFET in a particular LED driver IC.

As described previously, CSFB circuit 275 determines the lowest voltagepresent across the current sink MOSFETs within a given LED driver IC.This analog voltage is input to analog-to-digital converter 276.Analog-to-digital converter 276 can easily be realized using methodswell known to those skilled in the art. A 4-bit D/A converter 276 isshown in FIG. 13, comprising a voltage divider comprising resistors417A-417P (collectively referred to as resistors 417), a correspondingnumber of analog comparators 418A-418P, a stable source of a referencevoltage Vref 416, and a binary-coded decimal BCD digital encoder 420.

As shown, the reference voltage Vref is divided into sixteen linearlyuniform steps ranging from one-sixteenth of Vref up to Vref. Thesesixteen reference voltages are connected to the negative inputs ofanalog comparators 418A-418P. For example, the more positive terminal ofresistor 417A, i.e. the side not connected to ground, is connected tothe negative input of comparator 418A. Similarly, the more positiveterminal of resistor 417G is connected to the negative input ofcomparator 418G, and so on. The input to comparator 418P is tieddirectly to the reference voltage Vref. The positive inputs of analogcomparators 418A-418P are connected to the input terminal of A/Dconverter 276, which in turn is connected to the output of the CSFBcircuit 475. Because comparator 418A measures the lowest voltage of theseries resistor chain 417, its output can be considered as the leastsignificant bit or LSB of the converter. Conversely, because comparator418P measures the highest voltage, i.e. input higher than Vref, it canbe considered the most significant bit or MSB of A/D converter 276.

In operation, an analog voltage output by CSFB circuit 275 is comparedagainst the sixteen reference voltages at the negative inputs to analogcomparators 418A-418P. Powered by Vref 416, the individual referencevoltages are generated using a series string of resistors 417. For anygiven input voltage, the A/D converter input may exceed the referencevoltage on some comparators and fall below the reference voltage onothers. For those comparators where the CSFB input exceeds the referencevoltages, the outputs of the corresponding comparators will exhibit alogical “high” state. For those comparators where the reference voltageexceeds the CSFB input, the outputs of the corresponding comparatorswill exhibit a logical “low” state. For example, when the input voltageto A/D converter 276 just slightly exceeds the reference voltage inputto comparator 418G, then all the outputs of comparators 418A-418G willbe high and all the outputs of comparators 418H-418P will remain low.

In this way, the outputs of the sixteen comparators 418A-418P produce aunique digital combination of bits, i.e. of “ones” and “zeroes”,representing a digital approximation of the analog CSFB voltage outputof CSFB circuit 275. The sixteen outputs of comparators 418A-418P arefed into BCD decoder 420 that in turn outputs a four-bitbinary-coded-decimal or BCD code that is subsequently stored in samplelatch 277 as digital CSFB data. BCD encoder 420 converts the sixteenpossible combinations of the comparator 418 outputs into sixteen 4-bitwords in a one-to-one correspondence. One possible conversion code isshown in Table 2 below:

TABLE 2 High Low Comparator BCD Encoder Comparators Comparators Out Outnone 418P to 418A 0000 0000 0000 0000 0000 418A 418P to 418B 0000 00000000 0001 0001 418B to 418A 418P to 418C 0000 0000 0000 0010 0011 418Cto 418A 418P to 418D 0000 0000 0000 0011 0111 418D to 418A 418P to 418E0000 0000 0000 0100 1111 etc. 418M to 418A 418P to 418N 1111 1111 11111100 1000 418N to 418A 418P to 418O 1111 1111 1111 1101 1100 418O to418A 418P 1111 1111 1111 1110 1110 418P to 418A none 1111 1111 1111 11111111

The 4-bit DSCFB data in sample latch 277 is passed to the SLI busregister 280 as required, being subsequently shifted through the SLI busand into the interface IC in a manner described previously. Sample latch277 containing the DCSFB data is referred to as a “sample latch” becausethe process of converting analog data into digital data takes a finiteamount of time, i.e. A/D conversion is not instantaneous, so that thevoltage data is only “sampled” at some periodic basis. Moreover, asdescribed above, there is normally no compelling need or benefit tosample the CSFB feedback voltage in a video backlight system at afrequency significantly above the frame rate, i.e. at a rate faster thanfive times the Vsync frequency.

In the case of the prefix multiplexed SLI bus, the DCSFB data in samplelatch 277 is passed into SLI bus data register 280 through multiplexer278 in response to the corresponding prefix code for reading the DCSFBdata, where the prefix is decoded and the data is copied from the samplelatch into the data field of the SLI bus shift register. In a preferredembodiment of the prefix multiplexed SLI bus where the SLI bus dataregister is 16-bits wide, the 4-bit DCSFB word generated from the LEDdriver IC is preferably loaded into the 4 least significant bits of thedata register as shown in Table 3:

TABLE 3 DCSFB sample latch SLI bus data register wxyz 0000 0000 0000wxyz

In contrast, the fat SLI bus protocol, the data in sample latch 277 mapsdirectly into the corresponding 4-bit word in the SLI bus protocolwithout the need for intervening multiplexer 278. As such, each LEDdriver IC regularly generates at least one DCSFB word per IC and loadsthat information into the SLI bus data registers on a regular periodicbasis or upon request.

Since each driver IC generates it own DCSFB signal representing thelowest drain voltage of the current sink MOSFETs in that particular IC,the interface IC must sort through the digital codes to identify thevalue of the lowest CSFB voltage for all channels and driver ICs. Asshown in FIG. 14, this lowest CSFB voltage stored in digital register289 is then converted back into an analog feedback signal by D/Aconverter 290 and output as an analog feedback signal 292. The voltageoutput of D/A converter 290, named CSFBO, may be used directly to drivethe feedback input of a SMPS or alternatively operationaltransconductance amplifier OTA 291 may be used to convert this voltageinto a feedback current ICSFB.

Four-bit digital-to-analog converter 290 can easily be realized usingmethods well known to those skilled in the art. One such method,illustrated in FIG. 14, is to employ an R/2R ladder design comprisingresistors 431-438. Each digital input D0-D3 is biased in a logic “high”state at Vcc, or in a logic “low” state at ground. These inputs connectto the resistor “ladder”, producing a voltage V_(R) across the resistornetwork. By changing the binary bit combination in register 289, theresistor ladder voltage V_(R) may be varied dynamically. Specifically,if register 289 comprises a 4-bit word, sixteen possible digitalcombinations create sixteen unique equivalent circuits, each having adistinct and unique V_(R) voltage. To avoid interactions with varyingload impedance, voltage follower 439 buffers the ladder output voltage.By using resistors binary weighting, D/A converter 290 produces a linearmonotonic conversion of digital code into an analog voltage.

Supporting Multiple SLI Bus Embedded CSFB Signals

As described previously, one of the limitations of analog current sensefeedback is its inflexibility to support multiple independent feedbacksignals. Multiple feedback signals are required when more than one SMPSis required per system, either to support higher power levels or todrive multiple strings of LEDs having different colors. For example twoSMPS modules generating independent +V_(LED) supplies are needed inlarger, brighter displays where a single power supply is excessivelylarge and energy inefficient. In RGB backlighting, at least threeseparate power supplies, one to power strings of red LEDs, one to powerstrings of green LEDs, and another to power strings of blue LEDs arerequired. In some cases, in RGBG backlights four power supplies areemployed because two rather one string on green LEDs are needed toachieve the best color balance. Regardless, in present day systems,supporting multiple power supplies with different output voltagesrequires duplicating or triplicating the entire LED backlight system,making the solution costly, complex, and sensitive to noise coupled intothe multiple analog feedback signals.

Modifying the SLI bus embedded CSFB method to support multiple DCSFBsignals resolves the problematic issues of multiple analog current sensefeedback signals with no change to the BOM system cost other than theextra SMPS modules, no change to the SLI bus architecture, no change tothe LED driver ICs, and minimal changes to the interface IC. As such, asingle backlight system made in accordance with this invention can beadapted to support multiple supplies in a straightforward manner.

As illustrated in FIG. 15, a multiple power supply LED drive system 450made in accordance with this invention comprises a single interface IC451 and single SLI bus daisy chain 161 controlling an array of eight LEDdriver ICs 174 and two SMPS modules 453 and 456 with independentfeedback lines 452 and 455 for each SMPS module using the disclosed SLIbus embedded CSFB method adapted for multiple DCSFB signals. In thismanner, a single scalar video processor IC 153 and microcontroller 152can drive separate LED backlight arrays from two high-voltage powersupplies, each of SMPS modules 453 and 456 operating at the optimumvoltage for the LED strings it is driving. For precise current matching,all LED driver ICs 174A-174H share a common Vref analog referencevoltage on line 155.

In the dual-supply backlight system 450 as shown, LED strings 156A-156Hare powered from a common high-voltage supply rail 454, dynamicallyregulated to a voltage +V_(LED1) as a function of digital feedbacksignal DCSFB1. Interface IC 451 determines the value of DCSFB1 byinterrogating the SLI bus embedded CSFB data retrieved from LED driverICs 174A-174D and selecting the lowest value in the data stream. Thisdigital value is then converted to an analog feedback signal on line 452controlling SMPS1 module 453, either as a voltage feedback signal CSFBO1or as a current feedback signal ICSFB1. Similarly, LED strings 1561-156Qare powered from a common high-voltage supply rail 457, dynamicallyregulated to a voltage +V_(LED2) as a function of digital feedbacksignal DCSFB2. Interface IC 457 determines the value of DCSFB2 byinterrogating the SLI bus embedded CSFB data retrieved from LED driverICs 174E-174H and selecting the lowest value in the data stream. Thisdigital value is then converted to an analog feedback signal on line 455controlling SMPS2 module 456, either as a voltage feedback signal CSFBO2or as a current feedback signal ICSFB2.

Both DCSFB1 and DCSFB2 data are derived from interface IC 451interrogating the data stream on the SLI bus 161 which is shifted intointerface IC 451 via the SLI bus line 1611 and sorting through theincoming bits to determine the portion of each word constituting theCSFB data and the driver IC from whence it came. In the case of the fatSLI bus protocol, this sorting function can be achieved using a counteror with programmable logic to determine which LED driver IC relates tothe data that is arriving on line 1611. This method for encoding CSFBdata into the SLI bus protocol facilitates controlling multiple powersupplies from LED driver ICs embedding one-single CSFB feedback signalper driver IC. While driver ICs 174 do not require modification fromthose previously described and used in single power supply applications,such as the system 170 shown in FIG. 6, interface IC 451 in FIG. 15 mustbe modified to interrogate and parse its incoming CSFB data into twochannels of feedback in order to independently control multiple powersupplies.

A method of separating the CSFB data that relates to LED driver ICs174A-174D from the CSFB data that relates to LED driver ICs 174E-174H isillustrated functionally in FIG. 16. An SLI bus data stream 473 enteringSLI bus shift register 474 comprises eight CSFB signals 473A-473Hgenerated from and within eight separate LED driver ICs (not shown),with each driver IC sending its own unique CSFB feedback value to theinterface IC for processing. SLI bus shift register 474 which interpretsthe incoming CSFB feedback data, resides within aforementioned interfaceIC, for example within interface IC 451 or equivalent, which as shown,outputs feedback signals 487 and 488 to control two power supplies.

Since fat SLI bus protocol 471 containing CSFB data 472 contains noinformation regarding which driver IC generated the specific CSFBsignal, a counter or programmable logic must be used to identify thesource of the data as it is shifted into SLI bus shift register 474. Inthe example shown, the CSFB data words 473A-473D generated from LEDdriver ICs 174A-174D control the output voltage of power supply SMPS1while CSFB data words 473E-473H generated from LED driver ICs 174E-174Hcontrol the output voltage of power supply SMPS2. Digital word 471,which structurally represents each of words 473A-473H, comprises a CSFBpacket 472, representing CSFB data, embedded within bits constitutingnon-CSFB data. In the example shown, data words 473E though 473H, usedto control SMPS2, are shifted into SLI bus shift register 474 first,followed by the words controlling SMPS1. This sequence is arbitrary andcan vary from system to system. In fact, the data controlling SMPS1 andSMPS2 could conceivably be interspersed further complicating the sortingprocess.

To parse the incoming SLI bus data into distinct words 473A-473H,isolate the CSFB data packet 472 in each word, and identify which LEDdriver IC channel sent the data, a counter 477 counts the number of SCKpulses and a decoder 478 interprets what to do with the correspondingdata, either to load it from SLI bus register into an active latch asCSFB1 data, CSFB2 data, or to discard it. Specifically a multiplexer 475directs SLI bus data for channels A, B, C, and D to compare1 register478 and data for channels E, F, G and H to compare2 register 479. WhileSLI bus shift register 474 contains the entire fat SLI-bus protocolword, either 66 or 88 bits in length, only the 4-bit-wide CSFB packet472 is loaded into compare registers 478 or 479. All the other bits arediscarded or used by other registers and functions within the interfaceIC.

When channel-1 CSFB data packets 472 have been loaded into compare1register 478, the incoming data is compared against the data in “lowestCSFB1” register 480, overwriting the register 480 data only if the newdata has a numerically lower magnitude value. Otherwise the data in“lowest CSFB1” remains unaltered. At the completion of the SLI bus shiftoperation or upon the next Vsync pulse the DCSFB1 data is then loadedinto active latch and D/A converter 482 and the output voltage of SMPS1changes. The analog feedback signal 487 may comprise voltage outputCSFBO1 or current ICSFB1 where operational transconductance amplifierOTA 484 converts CSFBO1 feedback voltage into ISCFB1 feedback current.In a similar manner, when channel-2 CSFB data 472 is loaded intocompare2 register 479, the incoming data is compared against the data in“lowest CSFB2” register 481, overwriting the register 481 data only ifthe new data has a numerically lower magnitude value. Otherwise the datain “lowest CSFB2” remains unaltered. At the completion of the SLI busshift operation or upon the next Vsync pulse the DCSFB2 data is thenloaded into active latch and D/A converter 483 and the output voltage ofSMPS2 changes. The analog feedback signal 488 may comprise voltageoutput CSFBO2 or current ICSFB2, where operational transconductanceamplifier OTA 486 converts CSFBO2 feedback voltage into ISCFB2 feedbackcurrent.

In this manner counter 477, decoder 476, and multiplexer 475 are able toparse and sort SLI bus data stream 473, extracting SLI bus embeddeddigital CSFB signal 472 from each LED driver IC to dynamically controltwo SMPS outputs via analog CSFB signals 487 and 488. The concept can beextended to three or more power supplies by changing decoder 476 andadding extra compare registers and D/A converters. As shown, data stream473 groups all the LED driver IC data controlling SMPS2 into successivewords 473H-473E, followed by all the LED driver IC data controllingSMPS1 comprising sequential SLI bus words 473D-473A. In otherembodiments of this invention, the CSFB feedback data for two or moreSMPS modules may be interspersed in alternating or random fashion. Thusdecoder 476 may comprise reconfigurable logic, field programmable gatearrays, or a small microcontroller core in order to flexibly adapt thesorting routine to varying sequences. Note that at startup, the CSFBregisters are initially loaded with the highest value. After that, themost recent CSFB data is updated in the SLI bus data, adjusting thepower supply voltage dynamically in perpetuity.

In an alternative embodiment in accordance with this invention,information needed to separate and assign the SLI bus-embedded CSFB datato one of several power supplies can be built-in to the protocol itself.In the prefix-multiplexed SLI bus protocol and hardware interface, onemethod for extracting and assigning embedded CSFB data using separateprefix codes as shown in FIG. 17. In this example, a prefix code and itsassociated CSFB data identifying the SMPS to be controlled by thedigital CSFB data is embedded in the SLI bus data stream generated byeach LED driver IC. By reading and decoding the prefix data, theinterface IC is then able to easily assign the feedback data to theappropriate SMPS.

As an example, SLI bus data stream 493 embeds two types of CSFB signals,specifically SLI bus word 491 comprising prefix code “prefix I” withcorresponding data CSFB1 for controlling SMPS1, and SLI bus word 492comprising “prefix2” corresponding to CSFB2 data for controlling SMPS2.In this embodiment, SLI bus data stream 493 comprises four words493H-493E controlling SMPS2 and four words 493D-493A controlling SMPS1.The SLI bus data stream 493 is sequentially shifted into SLI bus serialshift register 494 under control of the SCK signal, whereby the prefixcode is interpreted by prefix decoder 495, and multiplexer 496 directsthe data to the appropriate functional latch.

In the case where the prefix code identifies CSFB1 data, prefix decoder495 directs multiplexer 496 to load the CSFB data from the data field ofSLI bus register 494 into “compare1” register 478, as shown comprisingthe CSFB data for LED driver ICs A, B, C, and D. Compare1 function 478then overwrites the data in lowest CSFB1 latch 480 only when theincoming data in SLI bus 494 has a numerical magnitude lower than thedata currently residing in “lowest CSFB1” register 480, otherwise thedata in register 480 remains unaltered. After all the words in SLI busdata stream 493 are shifted into SLI bus shift register 494 andinterpreted, the data in “lowest CSFB I” register 480 represents thecurrent digital representation of CSFB1 data, i.e. DCSFB1. At that timeor synchronized to the next Vsync pulse, the DCSFB1 data is copied intoactive latch and D/A converter 482, generating analog feedback outputs486 comprising voltage output CSFBO1 or after conversion by OTA 484,current output ICSFB1.

In a similar manner, when the prefix code identifies CSFB2 data, prefixdecoder 495 directs multiplexer 496 to load the CSFB data from the datafield of SLI bus register 494 into compare2 register 479, as showncomprising the CSFB data for LED driver ICs E, F, G and H. Comparefunction 479 then overwrites the data in “lowest CSFB2” latch 481 onlywhen the incoming data in SLI bus 494 has a numerical magnitude lowerthan the data currently residing in “lowest CSFB2” register 481,otherwise the data in register 481 remains unaltered. After all thewords in SLI bus data stream 493 are shifted into SLI bus shift register494 and interpreted, the data in lowest CSFB2 register 481 representsthe current digital representation of CSFB2 data, i.e. DCSFB2. At thattime or synchronized to the next Vsync pulse, the DCSFB2 data is copiedinto active latch and D/A converter 483, generating analog feedbackoutputs 487 comprising voltage output CSFBO2 or after conversion by OTA485, current output ICSFB2.

In this example shown, the first four words shifted into SL bus shiftregister 494 are those associated with control of SMPS2, i.e. LED driverICs H, G, F, and E, followed by four words controlling SMPS1, in orderas LED driver ICs D, C, B, and A. With the prefix multiplexed SLI bus,however, the data stream is not limited to a particular sequence.Instead, the data sequence can intermix the feedback data for SMPS1 andSMPS2 in any alternating or random sequence. Furthermore, using adistinct prefix code for each CSFB signal, any number of CSFB signalscan be embedded into the SL bus data stream.

For example, the format can easily be adapted to support three powersupplies consistent with driving RGB backlighting systems, or to supportfour separate feedback signals useful in RGBG or RGYB backlightingapproaches. RGBG backlighting solutions employ strings of red LEDspowered by one SMPS, strings of blue LEDs powered by another SMPS, anddouble the number of green LED strings powered by two SMPS modules inorder to compensate for the lower luminance of today's green LEDs. InRGBY backlighting, yellow LEDs are included to extend the range of colortemperature. The same system may also be used in signage applicationswhere RGB, RGBG, or RGBY are used to produce the actual image ratherthan to generate a white backlight.

In summary, FIG. 17 illustrates that separate prefix codes can be usedto embed multiple distinct CSFB signals into a SLI bus data stream toproduce a flexible solution to managing multiple power supplies withoutthe need for analog feedback networks. An alternative approach is embedup to four CSFB signals into the data field of a singleprefix-multiplexed SLI bus word, partitioning one 16-bit SLI bus datafield into four CSFB 4-bit “nibbles”. Such an alternative approach toembedding CSFB data into the SLI bus protocol is illustrated in FIG. 18,where one SLI bus word 501 using a 32-bit prefix multiplexed or “slim”SLI bus protocol comprises a 16-bit prefix code 502E and four 4-bit CSFBsignals 502A-502D.

The data field of SLI bus word 501 comprises CSFB1 word 502A comprisesthe four lowest significant bits in the SLI bus data field, bits 0 to 3,CSFB2 word 502B comprises the next four higher significant bits in theSLI bus data field, bits 4 to 7, CSFB3 word 502C comprises the next fourhigher significant bits in the SLI bus data field, bits 8 to 11, andCSFB4 word 502D comprises the four highest significant bits in the SLIbus data field, bits 12 to 15.

Prefix decoder 503 decodes prefix 502E instructing multiplexer 504 totransfer all four CSFB words 502A-502D into compare register 505.Compare register 505 then compares the 4-bit CSFB1 data 502A in SLI busto the CSFB1 data in lowest CSFB register 506, overwriting the data inregister 506 only when the new data in 502A is lower. Compare register505 simultaneously compares the 4-bit CSFB2 data 502B in SLI bus to theCSFB2 data in lowest CSFB register 506, overwriting the data in register506 only when the new data in 502B is lower. Contemporaneously, comparefunction 505 compares the data in 502C to CSFB3 data in “lowest CSFB”register 506, and compares the data in 502D to CSFB4 data in “lowestCSFB” register 506. After data is shifted into serial shift register501, “lowest CSFB” register 506 contains the most current CSFB values.

The data within register 506 is then used to generate CSFB feedbackoutputs 511-518 using D/A converters 507A-510 and transconductanceamplifiers 511-514, either in real time or synchronized to the nextVsync pulse. The four least significant bits in register 506 representDCSFB data processed by D/A converter 507 and OTA 511 to produce CSFBO1and ICSFB1 outputs used to control SMPS1. Similarly, in successive 4-bitcombinations, register 506 contains DCSFB2 data used to control SMPS2,DCSFB3 data used to control SMPS3, and DCSFB4 data used to controlSMPS4. As described in a manner made in accordance with this invention,one multi-CSFB instruction in SLI bus 501 independently controls up tofour SMPS output voltages dynamically in real time.

An application of the multi-CSFB embedded SLI bus independentlycontrolling three SMPS outputs is illustrated in FIG. 19 whereby SLI busdata stream 523 comprises a sequence of nine SLI bus words 523I-523A,each following the quad-CSFB protocol represented by prefix multiplexedSLI bus word 521 with a corresponding data field “wxyz”. The data forCSFB1 is encoded in the four least significant bits of SLI bus words523G, 523D and 523A, while the remaining bits represent the highestdigital value possible, i.e. 1111 in binary or “F” in hexadecimal. Thedata in SLI bus words 523G, 523D and 523A therefore comprise the 16-bitbinary words in the format [1111 1111 1111 wxyz].

In a similar manner, the data for CSFB2 is encoded in the next fourhigher significant bits of SLI bus words 523H, 523E and 523B, while theremaining bits represent the highest digital value possible, i.e. 1111in binary. The data in SLI bus words 523H, 523E and 523B thereforecomprise the 16-bit binary words in the format [1111 1111 wxyz 1111].Data for CSFB3 is encoded in the next four higher significant bits ofSLI bus words 523I, 523F and 523C, while the remaining bits representthe highest digital value possible, i.e. 1111 in binary. The data in SLIbus words 523I, 523F and 523C therefore comprise the 16-bit binary wordsin the format [1111 wxyz 1111 1111]. As shown, no word in SLI bus datastream 523 contains CSFB4 data, so it remains at the highest voltageDCSFB value 1111.

SLI bus data 523 is shifted serially into SLI bus shift register 524,once decoded by prefix decoder 525 instructs multiplexer 526 to writethe appropriate 4-bit nibbles into “compare” registers 527, 528 and 529from SLI bus data 521 respectively, with CSFB1 data being loaded intocompare1 register 527, CSFB2 data being loaded into compare2 register528, and CSFB3 data being loaded into compare1 register 529 forsubsequent conversion in analog feedback signals 539-541. As shown,CSFB4 data is not loaded into any compare register and therefore doesnot influence any SMPS output. Such a three output decoding isapplicable for RGB backlighting applications.

One possible RGB backlight is shown in FIG. 20 where LED driver ICs562A, 562D and 562G control and provide one of CSFB feedback signal 564for SMPS1 554, powering red LED strings 560A, 560B, 560G, 560H, 560M and560N with a voltage +V_(LED1) on supply rail 557; where LED driver ICs562B, 562E and 562H control and provide one of CSFB feedback signals 564for SMPS2 555, powering green LED strings 560C, 560D, 560I, 560J, 560Pand 560Q with a voltage +V_(LED2) on supply rail 558; and where LEDdriver ICs 562C, 562F and 5621 control and provide one of CSFB feedbacksignals 564 for SMPS3 556, powering blue LED strings 560E, 560F, 560K,560L, 560R and 560S with a voltage +V_(LED3) on supply rail 559.

All three CSFB signals are digitally embedded into a single SLI busdaisy chain comprising SLI bus lines 563A-563J and are converted intoseparate analog feedback signals 564 by an interface IC 551,facilitating dynamic control of red, green, and blue LED supply voltages+V_(LED1), +V_(LED2), and +V_(LED3), respectively, in response toinstructions from a μC 552 and a scalar IC 553.

In system 550 each LED driver IC drives two strings of LEDs having thesame color and outputs a single CSFB value. In an alternativeembodiment, shown in FIG. 21, each LED driver IC 612 controls threedifferent LED strings, i.e. on red, one green and one blue, and outputsthree distinct CSFB signals. Specifically, LED driver IC 612A controlsred, green and blue LED strings 610A, 610B, and 610C respectively, LEDdriver IC 612B controls red, green and blue LED strings 610D, 610E, and610F, respectively, and so on. The last LED driver IC in the daisychain, driver 612F controls red, green and blue LED strings 610Q, 610R,and 610S respectively, Each driver IC 612 outputs its own CSFB3, CSFB2and CSFB1 feedback values for red, green and blue feedback and powersupply control embedded digitally in SLI bus 613.

Interface IC 601 interprets the embedded CSFB data carried by SLI busdaisy chain 613 and outputs three separate analog feedback signals 614to dynamically control SMPS1 module 604, SMPS2 module 605, and SMPS3module 606 to produce dynamically regulated outputs 607, 608 and 609having corresponding voltages +V_(LED1), +V_(LED2), and +V_(LED3). Inthis manner scalar IC 603, μC 602, interface IC 601 along with sixdriver-ICs 612 form a dynamically adjustable backlight system withindependent dynamic control of eighteen LED strings using SLI buscontrol without the need for multiple analog feedback loops.

For more than one CSFB signal to be embedded inside a driver IC morethan one CSFB signal must be generated within a LED driver IC. FIG. 22illustrates an eight-channel LED driver IC 651 integrating eight currentsink DMOSFETs 653A-653H, four independent CSFB detect circuits6544A-654D, four separate A/D converters resulting in a 16-bit SPI busword 670 embedding four separate CSFB signals CSFB1-CSFB4 providingindependent feedback control to four distinct SMPS output voltage+V_(LED1)-+V_(LED4).

In operation, current in LED strings 652A and 652B produces sensevoltages across current sink DMOSFETs 653A and 653B. CSFB circuit 654Athen determines which ever of these two voltages are lower, and outputsthe lower voltage to A/D converter 655A, converting the analog feedbackdata into a 4-bit digital word CSFB4. Similarly, current in LED strings652C and 652D produces sense voltages across current sink DMOSFETs 653Cand 653D. CSFB circuit 654B then determines which ever of these twovoltages are lower, and outputs the lower voltage to A/D converter 655B,converting the analog feedback data into a 4-bit digital word CSFB3.Likewise, voltage feedback from LED strings 652E and 652F determine thedigital value of CSFB2 output by A/D converter 655C and voltage feedbackfrom LED strings 652G and 652H determine the digital value of CSFB1output by A/D converter 655D.

The CSFB4 data is stored in the four most significant bits of samplelatch 666, with CSFB3 occupying the next four lower significant bits,CSFB2 occupying the next four lower significant bits, and CSFB1 fillingthe lowest four significant bits of sample latch 666. When instructed todo so by prefix decoder 667A and prefix code 669P, all 16 bits of datafrom preload latch 666 is copied through multiplexer 667B into the datafield 669D of SLI bus shift register 658. This data field therebycontains not one but four independent four-bit CSFB words CSFB4 throughCSFB1, as shown by data set 670.

These CSFB signals after being shifted into the interface IC ultimatelyset the corresponding voltage outputs of +V_(LED4) through +V_(LED1)supplies in a manner described in FIG. 18. Four CSFB signals are usefulin RGBG backlights where the two sets of green LED strings are poweredby separate SMPS voltages. Alternatively, all 16 bits need not be used.For example, in RGB applications, A/D converter 655A can be removed andthe 12 least significant bits of sample latch 666 can be used to controlthree separate power supplies, for example +V_(LED3) for red LEDstrings, +V_(LED2) for green LED strings, and +V_(LED1) for blue LEDstrings. By eliminating unused CSFB 654A with current sink MOSFETs 652Aand 652B, and also eliminating unneeded MOSFET current sinks 652D, 652F,and 652H to realize a three CSFB, three-channel LED driver IC, such anapproach is consistent with the system example shown in FIG. 21.

We claim:
 1. A method of generating a supply voltage for a plurality oflight emitting diode (LED) strings comprising: detecting a highestforward-voltage of the plurality of LED strings; generating a digitalcurrent sense feedback (CSFB) word representing the highestforward-voltage; storing the digital CSFB word in a data register of aserial lighting interface (SLI) bus at a first rate; shifting thedigital CSFB word along the SLI bus to an interface integrated circuit(IC) at a second rate, the second rate being greater than the firstrate; and generating the supply voltage for the plurality of LED stringsbased on the digital CSFB word.
 2. The method of claim 1 furthercomprising converting the digital CSFB word to an analog CSFB signal. 3.The method of claim 2 wherein generating the supply voltage for theplurality of LED strings includes generating the supply voltage based onthe analog CSFB signal converted from the digital CSFB word.
 4. Themethod of claim 3 further comprising providing the analog CSFB signal toa switch mode power supply (SMPS).
 5. The method for claim 4 whereingenerating the supply voltage for the plurality of LED strings includesgenerating the supply voltage for the plurality of LED strings by theSMPS.
 6. The method of claim 5 wherein each LED string of the pluralityof LED strings has a first end and a second end and detecting thehighest forward-voltage includes detecting a voltage at the first end ofeach LED string of the plurality of LED strings, the second end of eachLED string of the plurality of LED strings being connected to the SMPS.7. The method of claim 6 further comprising storing a digital word inthe SLI bus representative of a magnitude of current in at least one LEDstring of the plurality of LED strings.
 8. The method of claim 7 furthercomprising controlling the voltage at the first end of the at least oneLED string based on the digital word representative of the magnitude ofcurrent in the at least one LED string.
 9. The method of claim 1 whereinthe supply voltage is a first supply voltage, the plurality of LEDstrings is a first plurality of LED strings, and the method furthercomprises generating a second supply voltage for a second plurality ofLED strings based on a highest forward voltage of the second pluralityof LED strings.
 10. A light-emitting diode (LED) drive systemcomprising: a plurality of LED driver integrated circuits (ICs), eachrespective LED driver IC of the plurality of LED driver ICs including aterminal to connect to a first end of at least one LED string of aplurality of LED strings, each respective LED driver IC of the pluralityof LED driver ICs being constructed to generate a digital current sensefeedback (CSFB) value based on a voltage level at the terminal of therespective LED driver IC and store the digital CSFB value in a dataregister in the respective LED driver IC; a serial lighting interface(SLI) bus being constructed to serially shift data through the dataregister in each respective LED driver IC of the plurality of LED driverICs; and an interface IC, coupled to the SLI bus, to receive the digitalCSFB value from each respective LED driver IC of the plurality of LEDdriver ICs and identify a lowest digital CSFB value.
 11. The system ofclaim 10 wherein the interface IC includes a lowest CSFB register tostore a lowest CSFB value representative of a highest forward-voltage ofthe plurality of LED strings.
 12. The system of claim 11 wherein theinterface IC further includes a compare register, coupled between thelowest CSFB register and the SLI bus, to compare the received digitalCFSB value from the SLI bus with the lowest CSFB value and overwrite thelowest CSFB value with the received CSFB value responsive to thereceived CSFB value being lower than the lowest CSFB value.
 13. Thesystem of claim 12 wherein the interface IC further includes amultiplexer coupled between the SLI bus and the compare register. 14.The system of claim 12 wherein the interface IC further includes adigital-to-analog (D/A) converter coupled to the lowest CSFB register toconvert the lowest digital CSFB value to an analog CSFB signal.
 15. Thesystem of claim 14 wherein the D/A converter is coupled to anoperational transconductance amplifier (OTA).
 16. The system of claim 14wherein the interface IC further includes an output terminal coupled toat least one power supply.
 17. The system of claim 16 wherein theinterface IC is constructed to provide the analog CSFB signal to the atleast one power supply.
 18. The system of claim 16 wherein each LEDstring of the plurality of LED strings has a second end coupled to theat least one power supply.
 19. The system of claim 16 wherein the atleast one power supply includes at least one switched mode power supply(SMPS).